555mkII find a better....

I currently use them bridged powering a 4ohm load.I understand each side has to handle 2ohms but I never got the feel that they were reaching their limit nor was heat a problem.I was just wondering if another killer amp like this existed that I could get for 400.00 used that would improve handling in a 4ohm load.

Depends on the sub, but an Electron Kinetics Eagle has some of the best quality bass ever. There's currently one on Ebay for $400 buy it now (not mine), though it is an older A model versus the C I am more familiar with.

Which subs? You should remember that bridging a stereo amp cuts the damping factor (bass control) in half. I've owned (4) 555IIs, the three with Toshiba output transistors sounded better than the one I had with ADC transistors.
Just take a flashlight & look in the side of your Adcom, you'll be able to read the transistor brand name.
I used Adcom 555IIs and Parasound HCA 1200II THX amps to drive passive Snell Sub 550s with a Sumo Delilah crossover, mated to Magnepan 3.3Rs running full range. Both brands sounded good, but sounded better NOT bridged.
An Aragon 4004 MKII is equal to a pair of Adcoms or Parasounds, you can find them used for around $700.00