Same here. Get an electronic crossover with level controls.
There will a certin loss of quality when intoducing the electronic crossover, does your speakers have seprate crossover boards? That would be best. Ive owned the ksa300s and can tell you for biamping lows you can spend less and have just as good lows with normal impediances.You may just want ot sell the krell and go Gaint tube amp like the VTL 450's monos. I can tell you that the VTL 300's will give that 300s a run for its money on the lows Ive owned both but the krell is much cleaner, the new VTL 450's would really be cool. Hell i just brought home a 22,000 pair of Von Schweikerts no ampification as of yet...
Faced with a similar situation I purchased a Solid State amp with gain controls. I purchased a Yamaha MX1000. I have owned a number of SS amps some at much higher cost and in my opinion the Yamaha is better than any I have heard so far. A real steal at what they go for. I have it paired with a $5000 tube amp if that gives you any indication how much I like what it does in my system.
The VTL's are good amps. Some people will like the audio research better so on and so on. Those VTL 450's will smoke. When you Biamp your going to run into other problems like level matching and there will be a area where both amps overlap that might stick out pretty good.... Depends what your after, sounds like may want staggering lows with pristine tube like highs, good luck!
The Aerials already HAVE a passive x-over -- you don't need to add an extra, electronic one. Unless you're ready to disconnect the passive LOW pass and the BANDPASS and keep only the tweet's xover; in which case a simple line-level passive or active x-over will do the trick.
The sound will be invariably better when you connect the amps directly to the spkrs with the xover being before the amps (it's easier to xover a small signal). I'm sure Aerial will help you in with info on their xover.
Sound level: it's a matter of amp sensitivity level and in some amps it can be controlled inside. It shouldn't be difficult to find out: check out each amp's sensitivity, the manufacturers could easily tell you!