Well the only thing better than one 106 is two 106's That is too bad that you can't put some mufflers on it to try but I can see how that would be tough. I have the first cnc milled spacer on my and it runs very smooth so either one should work. I does sound like more of a canister issue though. You should have a very linear and powerful low end on that engine when it is set up well.
Pat,
You can call me anytime if you want to chat about the Ultimate. If you don't have my number, pm me and I will send it to you.