A little late to the discussion, but here it goes: I restored a double pair of Nines about 10 months ago and I can attest they are amazing. The absolute cleanest bass I have ever heard. I have been communicating with David Jensen (invaluable help to my restoration efforts and overall great guy-- Thanks Mr Janzsen) and also have the original documentation from KLH. One of the myths about the Nines, as far as I can tell, is the fear of very low impedance at high frequencies and possibly destroying less capable amps. The documentation clearly states that the Nines are mostly 16 ohms through most of the spectrum and increasing to about 30 ohms at the low end. Interestingly, that is the reason while tubes sound good with Nines, the impedance is benign to tubes which relies on transformers to match them, not because it can handle low impedances better. It is also a very benign impedance to SS amps.
After the restoration I tried mine on a Fisher X-202-B and eventually restored a 100 W/channel Bogen MO-100 with a pair of brand new 6550 tubes on each. They did sound great but I must acknowledge that at least for my ears a good SS amp is a much better choice. The bass is way cleaner and well defined, the mids sound great and the highs are crystal clear. The tube amps always sounded a bit muddy compared to the SS. Whether we like it or not, you have an output transformer at the amp driving a step-up transformer at the Nines. Saturation at low frequencies can be an issue and attenuation and distortion at the higher end can also limit things. I don't have upwards of $40,000 at the moment to get 4 Marantz 9s :-) , but the physics are the same.
Most importantly, as Mr Janszen and the documentation emphasizes, the real danger is to drive the speakers at voltages higher than about 26 volts, which translate to about 90 to 100 Watts for an 8 ohms load (a pair of Nines). Any higher and the bass panels may arc and cause damage to the membrane.
So to answer your original question, yes you can drive a double pair of Nines with any amp up to 100 Watts/channel (a pair per channel) all day long. Better to be close to 100 watts to avoid clipping. The original owner drove them with a Dynaco 120 solid state amp (60 W/channel) and eventually with an Adcom 545. I drive them with a BGW 250E at the moment which sounds surprisingly good.
Let us know if you kept the Nines. They are great speakers.