Getting my Poodle to Shut Up!

My poodle used to be the friendliest, loveliest poodle you could meet. Loved all dogs and people to death!
He was neutered later in life b/c I was hoping he could be a show dog for me but he grew to big (he was supposed to be a toy but he is 12" tall). He was never aggressive.
He was neutered at 2 and a half or so. Now he is such a frustrated and cranky poodle. We also got a doberman puppy early this year (it was supposed to be my brothers dog but his wife couldn't handle it pooping in the house).
In my grooming shop, if any body comes into the front the dogs bark and bark. Styder (my poodle) will sniff under our door that splits the front area from the grooming area. And if he sniffs out another dog under there he will sometimes growl and bark excessively loud. Once they come in and he is there he is happy and sniffy and playful with the little dogs.
Sometimes he will bark at the dogs on my table (specially the big dogs or dogs that are giving me some trouble) and if I bring a dog out to return it to it's owners he will growl and bark and nip at them.
Since he has been neutered his whole attitude has changed and since he has been being 'bad' he gets 'disciplined' a lot so I thought last week that maybe he is being so bad b/c he is frustrated with not having enough positive energy, and not having enough alone time to play without worrying about the dobie puppy stealing his toys and such. So I have really been trying to be very positive and happy with him. Giving him special play times to himself and making sure the dobie doesn't bother him.
The thing is that he will take his toy and deliberately go lay next to the dobie with it, then he gets mad at the dobie for even looking at him.

Things that I have tried in order to get him to stop:
Voice command of quiet or eh or something
Making him do commands such as sit or down to distract him and assert myself to him and if he is distracted by the people or whatever and doesn't do the command he gets a collar correction.
Ignoring it.
Giving him treats if he doesn't do it (he might shut up one time and gets a treat for it but with go crazy with the next person)
Putting him away in a back room or kennel everytime he does it. He will run happily into the room if you open that door. We put him away in this room when we are bringing dogs out so he can't growl and nip at them.
Using positive voice and calling him over to reassure him that it is nothing thats gonna hurt him and that I am here.

The one that worked the best was to use a clapper that you get at sports events that makes a bit of a noise. They would still bark once or twice then you used that and they would stop. However that broke, and I can't use something too loud as I don't want to scare my client dogs

He gets about 40 minutes of offleash running everyday, though I don't notice too huge of a difference whether he has had it or not.
He knows a wide veriety of commands
sit (voice/hand command, just voice, just hand)
down (voice/hand, voice, hand)
Stand (voice/hand, voice, hand)
Come (voice/hand, voice, hand)
Distance commands of sit, down, stand (voice/hand, voice, hand)
Stay in sit and down well with lots of distraction and me going long distances.
Lots of fun tricks and such too.

He is stubborn but highly trainable to regular commands but he just won't shut up. He also grew up in a grooming salon for nearly his entire first year of his life! and he was good then!

If this is relevant we could never teach him to crate train. He always screamed and whined no matter what! We ignored him for months and months on end, never letting him out till he was quiet and he never learned. At the end we also tried other things such as shake can, 'shaking' the kennel, other noise disciplines. Nothing worked.

Sorry this was so long. If you need any other info just ask.

First of all, the personality change after the neuter could very well be just your dog getting older, not actually caused by the neutering. It could also be caused by the puppy, who knows. At this point, the cause doesn't really matter.

Second of all, I'd never suggest or do collar corrections on a small dog, particularly a poodle. Being a groomer I'm sure you know that poodles often have collapsable tracheas, and collar corrections could seriously hurt them.

It sounds to me like he has way too much freedom. He should not be given the opportunity to sniff under the door if that causes him to bark. He should certainly not be given the opportunity to bark, growl, and nip at dogs walking through the salon (and if I were that dog's owner, I don't think I'd come back to a salon where my dog got nipped at by the groomer's dog!). Since unfortunately you can't crate him, maybe you could set up a tether so that he can still be in the salon, but not have the freedom to practice these bad behaviors. Next, if he starts vocalizing at another dog, IMMEDIATELY remove him from that dog's sight, so that when he vocalizes, he no longer gets to look at that dog. If you can set up a barrier (curtain, wall, whatever) to put in front of him or to put him behind, that would be good. Otherwise, move him to the back room or somewhere else. The key though, is to do this QUICKLY and without emotion; it will be best to keep him on a leash if you have to remove him from the room, so that he has no choice in the matter.

I'd also highly suggest the book "Click to Calm" by Emma Parsons, I think the methods described in the book will help your dog tremendously.

Collar corrections work just fine for mypoodle.

He does not get to bark or nip at the other dogs leaving my shop as he is put away (which I stated in my OP). And like I also said in my OP, we have tried to put him away everytime he starts to bark and it has not helped.
Thanks for the reply.

Instead of all out putting him away, if he barks and you simply move him away with his leash each time he barks (move him from the door) until he stops that might work. I also like Lizzy's suggestion of opening the door, with the dog on leash and closing it each time the dog barks (also have him far enough away so that he cannot shove his nose under the door). If he is quite for a time open the door again, he'll learn he can watch the dogs if he is quite. I also think it's important that with any method, including the one where you used the loud noise to startle him you make sure to reward him with food for remaining quite. If there is no benefit of being quiet why do it?

I am against collar corrections as much as I am against hitting, and they can cause throat problems in small dogs, collars gave my relatives dog a collapsing trachea. But I am not against corrections just for the health issues, but because of the psychological damage such punishments can cause to the dog.

Then why is he still barking?

Get professional help. There can be a variety of reasons the dog is barking and for the personality change. You'll save you and your dog a lot of further aggravation if you have an assessment in home to troubleshoot the issue. Try http://www.iaabc.org/