As I said, don't get hung up on the amplifier topology -- this is all but irrelevant to the tone when comparing one solid state type to another; the preamp section counts way, way more, and is what lends each amp head its "voice".
While I do use my RH450 for upright, I am definitely happier (for now at least) with my Markbass F1 (with the fdeck HPF-Pre in front). I would neither recommend nor discourage the use of either amp for upright, but they're what I have.
Your speaker cabinet, your bass, your pickup, your technique, and where you play all contribute greatly to what comes out the far end, and it's really difficult to suggest anything that absolutely will work for you, no questions. Most will suggest one of the more "neutral" sounding amplifiers (and have done already in this thread), but sometimes an amp's particular voice will complement the rest of your setup and your playing in predictable ways. I for one, will make do with what I have until I can actually play my next amp with my gear; there are just too many variables to recommend otherwise, in my view.
All opinions to the contrary welcomed and respected, of course!