Hummmmm..sounds to me like you are the one after the combo!, ey?
Anyway, I've sold this combo on a retail level over the years, and am very familiar. I've had the 750 in my system before.
The 5802 is very sonically comparable to the ATI amps, if slighly less extended in the trebble. Otherwise, very same class amp. The amp and preamp in mention are descent class B components. Actually, the preamp (GFP750) I think is at best borderline Class B/C. I think the class A rating...,well, someone got paid off there...that's what I think.
Sonic weaknesses between the two pieces?...well the preamp isn't as pure as proposed. It's a little syrupy, and not as pure as better "passive pre's", like the Pass Aleph L/P!!! Compared to those, the GFP750 is more veiled, syrupy, and not so pretty sounding compartivley(again, the review lied!)
It's a descent preamp however, but nothing superspecial. The 5802 amp is slightly light on dyanamics and bass authority, relativley clear/uncolored, with descent midrange, and ok highs. Soundstage can be bettered, as can air, ultimated detail, etc. AGain, it's Class B. Still the best amp Adcom ever made mostly.
However, that said, price per price, it depends on your speaker needs! What will those be???
Anyway, it's good gear. If you like the 750's sound, you'll find no fault in generall with the 5802 I think. Again, they're good components that offer good sound overall.
However, my recommendation is to figure the speaker chioces first! The speakers are giving you the biggest sonic signature, and are by far the most important component in the gear-chain. I'd figure those, then match the components...or in this case, the amp. That would make your amp chioces easier. Heck, some speakers do better with tubes!
Anyway, I don't understand falling in love with an amp/preamp(unless you just like to sit around and show your friends your toys) combo without knowing your speakers!...that's backwards..and I know so many people do it that way though.