First, bear this in mind:
Shackman = bad. Keep him as far away from the show as humanly possible.
Jamie and Jonah = VERY GOOD
Really, with the exception of Shackman, I think the key is to have more than just Marc and Lowell on the show. That may sound like a dig on Marc and Lowell, but when it is a group all feeding off of each other, it is a funnier show. When it's just two guys talking to each other, it isn't as funny. Look at it this way -- Don and Mike were never just "Don and Mike", they always had other voices, whether it be Buzz, Robb, Sherry, etc., to add to the day. In the end, Don and Mike were a great pair (just like Marc and Lowell), but they were made greater by the group atmosphere. That's one of the reasons TMOS is so amazingly great -- Mike O'Meara is the show, but the show isn't just Mike.
Keep it up; it is great to hear from the two of you, and I hope to hear lots more for years to come!