I would classify Adcom as mid-fi, so yes the Mac amps are a HUGE upgrade.
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I used to drive my original FOCUS with the 565's and I felt they did an outstanding job especially on the low end. I moved on at a point to several different amps and finally ended with Bryston 7Bst for several years. I still felt in some ways the Adcoms were the best on the low end but the Brystons were still just overall better. Now I own a pair of FOCUS SE and started with them using the same Bryston. Sounded great. Then I fell into a good deal on a pair of Mac 352's that I bridge and they took the FOCUS to another level. My first Mac and I couldn't be happier. At this point I likely would never change amps as I have never had the same synergy as I do right now so I find it hard to believe it would get better and likely would be worse if I did something else.
With that said I don't know the 754's but I surely can say I love the sound of my Mac on the FOCUS. I did find the preamps made a difference as well so that is part of the equation.
I owned the adcom 555 then moved on to the 565 mono's. For the price they did the job however when I bought a pair of PBS stratus golds the 555 couldn't keep up. Unfortunately I had already sold the 565's. The amp would shut down (go into thermal overload) after little use at even lower volumes. I was just getting into audio so I didn't understand speaker loads and efficiencies. The stratus golds were not a good match for the adcom. I agree with Swin1 in that the bryston is overall better and I would even go further and say that the bryston is better than the adcom on the low end as we'll. I also owned several brystons and always felt they were heavy on the low end. I did have the opportunity to own a Macintosh solid state amp years ago ( can't remember the model) and it totally blew away the adcom. I remember listening to cd's and hearing detail that I had never heard with the adcom...using the same preamp, CD player and speakers.