Amp Upgrade - 3k to 4k used?

Hi Folks:
I’m thinking of upgrading my amps and would value some advice. I currently have Blue Circle BC2 Monos. I’ve been really happy with them, but the upgrade bug is starting to bite again. I could spend about $3k to $4k on the used market. I’m particularly keen on tube amps (especially low powered SET), but not sure if my speakers would be efficient enough. I know my CD player is a little old too, but don’t think an upgrade here will get me a big bang for the buck. I’ll also consider upgrading IC and SC as well a little later on, but want really really good stuff (FIM or Valhalla likely). Another option may be to get a Shunyata Hydra. The rest of my system is:

Audio Physic Virgo II (89Db efficiency, I think)
Sonic Frontiers Line III – (but expecting delivery of Supratek Chardonnay in a month or so).
YBA CD 1a
Nordost SPM IC & OTA Sakura IC
OTA Sakura Speaker Cable
3 Shunyata Sidewinder Power Cords, 2 YBA Diamond Power Cords, 1 pair of Bybee IC Filters, FIM outlets.

All advice and recommendations welcome. Thanks,
John.