Adcom gfa 555 or 5400

My experience with Adcom amps is kind of dated (late 80s to early 90s) but I did have access to their line as a salesman and purchased several models on accommodation during that time.  My memory of those two amps specifically are, the 555 had good power, decent sound stage but somewhat limited air around musicians on stage, upper mids into highs tended to get a bit bright when asked to open up the volume.  For all that power, I always found myself turning it back down to tame the edge that would start to appear and this was on many brands of speakers.   The 5400 came a few years later on, still had a few pairs of the speakers from the first go-round with 555, 565’s & 585-SE.  The 5400 seemed well balanced, smoother mids and highs with less grit or sizzle, sound stage was admirable for a budget amp with better spacing around musicians.  The only slight down side was the bass when volume was elevated enough to demand more control and reserves.  This is where the 5400 would run out of gas compared to the older big block style amps.  
I don’t listen anywhere near as loud as I used to and if given a choice between those two amps now I would lean on the 5400 for a more musical presentation.  

I had a 535, a 545, and a 555. I thougt the 535 sounded best.  Im not so sure though that Adcoms are going to pair well with your monitor audios though.  The sound may go to lean and bright...

I fried my 535 by accidentally touching the positive and negative speaker wire leads together while the amp was on, by the way.
I always felt that B&K amps, especially the reference ones, to be the best sounding / bang for the buck amps in that $300-$400 range.

I’d look at B&K, Parasound and NAD amps in that price range too!

Thanks very much guys. Looks like I can get my hands on a parasound hca 1000a for about the same price. 

I had the 535 paired with the 400 (which I still use as a phono pre-amp and radio).   I more or less agree with the previous posters. At first I heard plenty of detail I wasn't used to w/ my old system, but then "lean" "too bright" "sizzle"--whatever. A lot of my music (particularly voice/choral) became unlistenable.  The best thing about that amp was that it inspired me (in some desperation) to try out an entry level tube amp, which I have been using ever since.  I gave the Adcom away.

I kept my 535 because it's not worth much and still sounds better than a receiver. I don't have a receiver, but if I inspire a friend to jump ship and go more toward audiophile gear, it's an easy re-gift! Good luck, OP!