My Meadowlark Shearwater Hot Rods have an efficiency close to your Virgos but are 8 ohm rated. I am using Shanling SP-80 monoblocs in stock form but with NOS driver tubes. This amp is the only one I know of which gets 50 real watts out of a pair of sweet-sounding EL34s. Mods are available from 2 or 3 places, looks are killer. Built-in remote-controlled preamp can be bypassed simply. MSRP $2495.
U.S. distributor's page
U.S. distributor's page