Continue to use Audyssey. The only thing adding an external amp will do is likely change the overall amplification factor on those channels since it will probably have a fixed gain that is different from the fixed gain in the power amp section of your receiver. So you will need to run Audyssey again so it can compensate for that. But it otherwise makes NO difference.
Speaker placement/angle and room treatments/placement of furniture/etc. are also important and could probably also be optimized for better sound if you haven't spent much time on that. But even if you don't do that, still use Audyssey (heck, Audyssey will probably be MORE important if you don't do the proper set-up work, since your system will likely require more correcting than it otherwise would). And even if you do optimize your room, I still recommend using (or at least trying) Audyssey.
Don't let him talk you into buying new cables, though, unless it's worthwhile stuff like upgrading video connections from composite/S-video/component to HDMI. Don't let him talk you into expensive speaker cables or anything like that. They won't make ANY difference at ALL, regardless of what he tells you. Since you did mention he mentioned cables, I assume he's going to try to convince you to buy a bunch of stuff you do NOT need, not even slightly.
"Vintage" is good for wine, not for A/V equipment.