Choice: Avantgarde Duo Omega or JBL K2 s9800

Art,
Both are world class speakers and offer much.
I only heard the JBL's in a demo room and not a home situation so my impressions are thus biased. Dynamics were remarkable and they sounded mostly well balanced but I did not have as much of a musicians were in the room illusion that I hear with my Duo Omegas- Bass was powerful and maybe even a little bit aggressive to my ears despite not going as low as the AG's (35Hz-JBL's vs. 20Hz-Duo's). The Omegas are a significant step up from regular Duos and the bass is not slow or poorly integrated at all as Triode points out.
Good luck with your decision.

I have heard both speakers many times (though never in the same system - perhaps that disqualifies me). Both are excellent. They have different flavors. I agree with Triode in that when they are well set up, they don't have 'poorly integrated' bass, but that it takes more work to set them up than it would the K2s. I don't think either will be regarded as particularly good financial investments but either will provide a great amount of satisfaction. For some reason, and I am not sure why, I think the K2 does all things at a very high level, and the AGs do most things at a slightly lower level of perfection but some things even better than the K2s (Inman29's 'illusion'). I also like bass a bit further down than the JBLs can go, but that's me. The K2s' bass sounds really, really great.

About the keeping the BAT pre/power, my personal inclination is to say that the AGs really shine with a SET amp. The K2s need more juice than the AGs and your BAT amp is probably enough for the K2s (though if I were starting from scratch, I'd tend to want a bit more than 55W.

One thing I forgot to mention above, for what it's worth to the OP...at one time (and it may still be the case), a couple of the 3 principals in BAT reportedly had Avantgardes in their home systems. So even with higher-powered BAT amps, there may be a synergy.

Art80342, I spent about a week listening to the K2, in a very controlled environment. My opinon comes from listening to both speakers, both with amps that I know and also those that I do not.

Either way, the K2 is an easier setup (it can be close to the rear wall), with greater coherence between the bass and the rest of the speaker (not surprising since the woofer is driven by the same amp that the midrange is). I found the speaker very easy to drive for 'only 93 db'. I think JBL is being conservative. The impedance curve is also easy for lower-powered tube amps. I found 60 watts to be nearly impossible to clip- literally so loud earplugs were required.

Atmasphere, I have no doubt that your K2 observations are accurate. But the ability to place them close to the wall is not a point of differentiation; AGs have sealed subs with the ability to adjust level and crossover point, and phase is easily reversed. Very flexible and adaptable.