How much to spend???..alway's a factor.
My suggestion on those speakers is to "biamp"! They are set up well for that I belive...at least the 3.3's and 2.9's were. You'll get much much much stronger, more dynamic transparency, slam, control, and more believable pressentation, from an otherwise, lower sensitivity, dynamically a bit shy sounding speaker! I suggest something like the Parasound on the bottom(parasounds usually have very good bass for cheap),then do somehting better on top (also the Parasound has volume control for matching up the amps!!!). An upgrade for there on top is, yes, Plinius, Threshold T series, Classe is an improvement(but not as huge), B&k's are about the same league as Classe(but not super on top), Sim's are good/better, actually the list goes on and on. You could try tubes on top, but don't know if you want to get that tricky...probably sound great though with a solid state on the bottom posts, driving bass, yes.
Anyway, these are descent enough speakers on their own. they're a bit bass boomy, and can do with some really good speaker placement chores(as all speakers).Also, you can get an EQ on the woofer in a biamping situation, leaving the midrange/tweeter unaffected.
You could futz around alot with those speakers, and get some really well balanced, dynamic clear sound. Those speakers aren't the best at retrieving all the detail, as the other gentleman stated..never allowing the speakers to dissapear totally. But, overall, you'll have fun upgrading the sound if you want to get more into it. Have fun....