I'd agree with Tvad - you won't get the improvements you are after for a $200 mod as described. I also don't think they'll come swapping tubes to any great degree, and great NOS tubes are pretty expensive and somewhat risky. What you describe as your goals, a fuller sound with better staging and bass, may indeed be realized going the route of a KT88 based tube amp. I've owned the 302B and have listened extensively to two friend's 502B's and I could tell you the fuller sound and better bass would be realized there. I don't know that the soundstaging ever occurred to me as any better via the KT88 Jolida. You might try listening to other options in the same ballpark (Cayin, Prima Luna, etc.). Don't rule out SS completely either...though they may not have the soundstage depth they can certainly achieve width and definition and will likely give you improved extension as well. There are tradeoffs either way. You just have to listen for yourself and see what works best for you.
