Audio Physic Virgo II - subwoofer recommendations?

I'm a long-time owner of AP Virgo IIs, which are currently being driven by VAC Std Preamp -> VAC PA 100/100. In my mind, it's just a great setup (very transparent), but lately, I've been thinking of adding a subwoofer to mix. What are people's experiences with adding a sub to the Virgo IIs for music-only? Lately, AP Lunas and Rheas seem few and far between on Audiogon - are there any other good options (preferrably <$2k)? Thanks.
I used to own your Virgo's. In some ways I still miss them. Heard the newer AP's at my dealer recently and that memory came rushing back.

Personally, I wouldn't do anything to mess up that sound. I'd wait until one of the AP subs shows up.

Good luck.
Rel sub bass units integrate really well with many different speakers. I have a Storm 3 and love it. They MUST be meticulously dialed in, but those efforts will be rewarded quite nicely. Quadophile has written an excellent and detailed review of the Rel Acoustics Strata Mk III on Audiogon:
Rel Strata Mk III Review by Quadophile
Virgos need a fast, articulate subwoofer to integrate with their speed. Zu Method would be perfect, and the only sub quick enough to really sound seamless with them.

Hi Yegg, I recently picked up an ACI Force sub for my AP Sparks, original version. Obviously, I don't quite have the bass output that the Virgo II's exibit. I really wanted a musical sub above all else, and the ACI subs fit the bill big time. The sub actually helps my soundstage out a ton. I would recommend the Titan, very solid american build quality, and not to mention easy on the eyes.