I've always been a fan of the Aragon "sound" ever since I picked up the 4004 MkII over ten years ago (I still have it). I just upgraded my "3-channel & receiver used as an amp" arrangement and acquired an Aragon 2005. It hasn't lost any of the Aragon magic, as far as I'm concerned - though many people dislike the "HT" orientation Aragon is taking their electronics with the 2000 & 3000 series amps, vice a "music" orientation (i.e., downgrading the power supply, the build quality, heatsink area, etc.). So far the 2005's held its own with the 4004. I am planning to upgrade to an 8008 MkII or a pair of the new Palladiums, but that isn't going to happen for awhile.
I'd say the top three (I'd include the Sherbourn on reputation alone) are very similar to each other - you'd be hard pressed to tell the difference. I've listened to my friend's ATI1505 on JM Lab speakers, and a B&K Ref200.7 at my local dealer with a variety of speakers (love those Piegas! - so does the better half).
I think the ATI's a bit more clinical - but it's hooked up to some pretty forgiving speakers - but it's a great amp. Detailed, a great spectral balance, except for a little more upper treble emphasis than I'm accustomed to. The B&K is a bit warmer, probably more so than the Aragon (the 3-channel amp I sold was a B&K ST1430). It seems a little less fatiguing than the ATI, but not much so.
You really can't go wrong with all three, but they're a bit on the $$$ if you're just getting started in separates. In fact, the ATI 1505 & B&K 7250 were my alternatives on the 5-ch upgrade until I found an open box 2005 for sale. No opinion on the Sherbourn or the Anthem (really haven't listened to them closely).
The Adcom, in my opinion, loses just a little refinement, at least over the Aragon - but I'm going on the older models of Adcom. What you get is the best bang for the buck, bar none. Now I've heard the newer Adcom amps (like the 7605 you're looking at), are redesigned by a very notable designer (forgot the name - from CAL?) so I'm sure they're outstanding, if their track record holds.
I'd suggest step in a little cautiously and audition the Adcom (and the Anthem if you can find one as well), and see if you like it. If you do, probably your best choice to start!
Welcome to separates!
Happy Listening!
Chris