The amp design of the 170DC is likely a little more aggressive than a 2285 receiver, and the build components a little beefier. It may, therefore, be marginally more reliable as comapired to an 2285 amp section. All that aside, any sonic difference you might hear will be more a question of relative condition, given the age of both, than any difference in amp design. In your comparison between the receiver alone and the receiver-preamp combo (pre-out into the 170DC), at a moderate volume, do you notice a sonic difference? Likely, you will get better performance at a higher volume with the 170DC, but only marginally until you demand more of the 2238 receiver than it can comfortably deliver. The loud transients will suffer first at higher volume with the receiver alone.
IMHO, the 170DC is a very nice 85W/ch RMS amp. Its a keeper. Use it as you have it set up until you can add a preamp you like. Here the main issue is the input sensitivity of the 170DC (probably 1.5 volts for full out put of the amp. Techs, correct me here).
Reliability, build quality, ease of repair, and lastly esthetics matter, in that order. IMHO.
In the end, do you care if they match? There's your answer. Its certainly not necessary, though. If you do want them to match, ChrisMarantz gave the answer, above.