Help power pair of Infinity Kappa 9's please

Excellent thread!  Another new 9Kappa owner here.  I'm mainly trying to figure out what subwoofer to get and what preamps/crossovers I will need to try bi-amping.  But I'll tell the longer story here just for fun:

I always loved the look of the Kappa 9s -- plus it is possible for them to sound great with the right amps.  After 30 years of wanting a pair, someone a mile away had an estate sale over the summer and I ended up with a pair of Kappa 6s and 9s (all 6 woofers in desperate need of foam) as well as a Nakamichi PA-7, Adcom 535, and a LaserDisc player.  Quite a nice rig in the late 80's!

I knew what I was in for with the Kappa 9 amp killers, and I have been holding on to a Parasound HCA-3500 just for this occasion -- it is probably a good "starter amp" for these monsters.  I expect there will be a lot of tweaking ahead to get the best sound, and just to make things more tricky -- I have never used dipole speakers before.

I just got the refoamed woofers back yesterday!  My first impression was: it sounds terrible when one speaker is on Extended and the other isn't.  Once I figured this out, it starting sounding better -- but I am definitely going to want a subwoofer since Phish bass is hugely important to me.  Can someone recommend one that would be a good match and in line with the "entry level audiophile" price range I usually shoot for?  My previous speakers were Boston VR-970s which I really enjoyed -- the low end on them was perfect for live rock concerts.  If I can get anywhere close to this type of low end in a way that matches the Kappa 9s, I will consider this upgrade a success.

Also, this is my first bi-amping experience so I am not sure what kind of preamp/crossover gear I should buy to start experimenting.  I think it will be best to use the PA-7 for the high pass and the 3500 for low pass... plus the subwoofer.  Presumably I will want a crossover that will split into high, low, and sub levels from the 2 preamp channels, right?  Is there any way to do this that will still allow 5.1 to work properly -- i.e. an integrated preamp/crossover?  This is starting to feel like a "lmgtfy.com" moment now.  :D

Thanks!Mark
p.s. is there any point in trying to understand the details of Extended mode?  If I add a sub and I'm bi-amping is there any reason to choose one mode over the other?
p.p.s. wow at moderate volume the HCA3500 is too hot to touch for more than a second or two... might be a fire hazard.

Quick update: what makes the sound even worse than mismatched "Extended" modes is when the bottom woofer is wired backward. I re-did the wiring on both speakers and it sounds infinity-ly better now. This seems to be a common problem on these speakers -- I saw a surprising number of "refoamed and sounds crappy" threads.

I still think I’ll want a sub, but obviously the need isn’t quite as great as it was an hour or two ago.

Oops I was posting on two different threads.  This must be a popular topic!
The bottom line: I figured out how passive bi-amping works and it sounds wonderful.
The other important piece of information I figured out: the best place to go for all your bi-amping and RCA cable needs is a car audio shop.  They are big time users of crossovers and RCA cables.

On the other hand for costs you could try two Threshold SA3, Two pairs of Carver Silver 9T's, two Threshold S300 or a Mark Levinson No.23.

The best results I have gotten with tube /hybrid preamps. If you cannnot afford expensive ones try Audible Illusions Modulus3 or Mfa Magus.

I hope this will get the hassle out of bad reviews.

I have a pair of the K9's -I bought them new 1988 !

I have matched them successfully for over 30 years with a a pair of Metaxas Solitaire high current power amplifiers.

The speakers and amps are in storage pending my return to Australia- Locked out for 3 years due to covid.

I am going to restore the Solitaires back to factory spec  and get these babies back singing.