Since you have acquired all your major audio pieces, I think you need to be very goal-oriented in selected that last few. What does your system not do, that you would like it to do.
There are quite a few top level preamps, and each has different strengths and flavors. I would imagine (don't know though) that the WAVAC would provide a more intimate, rich, and larger image size with great midrange texture. The BAT is very taut, clean, and dynamic with great bass and ability to soundstage large orchestral works. Another consideration is whether you care to have XLR or RCA interconnects.
Other preamps to research include Joule, Lamm, and VAC.