Bi-amped system

I remember those speakers. I liked them.

If you want more midrange bloom I would look at your preamp and source. Although, someone can maybe recommend some warmer tubes. 
 

We can probably help more if you show your system under your UserID. It is quite possible there are other ways to enhance the midrange. A sub-woofer will not help.

How loud do you play your system? How far off the front wall are the Virgo’s? How far are they from the side walls? How big is your room? Have you thought about using a SS stereo or a different tube amp? My memory say’s those want more power than your Citations can produce. 

KT88 and 6550 are pin compatible. EL 34 can only be used if the sockets are wired for triode mode operation(grid 3 to cathode, pin 1 not connected as with KT88/6550).

I use the EL 34 for driving a Lowther in an OB (400-20kHz). The same amp sounds best with the 6550 connected to  front loaded horns (400-4.5kHz).

The KT 88 sounds more silky in the upper mids with a wider image, the EL 34 has definitely some coloration but sounds very warm with wonderful sound colors, the 6550 sounds very dynamic and clean with more punch. Very nice for driving an  active  KH bass.

All 3 tubes have a different plate resistance so the sound in the Citations  might also change  because of this but you could influence it by connecting the speakers to a lower or higher output impedance (if the speakers are easy to drive ) which will change the impedance of the OPT primary as well.

Each tube will sound act differently in different circuits, so you really need to experiment a bit. Some of the best bass I’ve ever heard (tubes or SS) was from a set of CAT amps using KT120s. In my Dyna/VTA 70s, the Nesstone 6CA7 did better with bass on the woofers in a vertical bi-amp system than the EH-EL34s, EH 6CA7, Shuguang, or Siemens EL-34s, IMHO. It’s subjective, room dependent, and system dependent too, so YMMV.