Joule-electra LA-150 Mk 2 dead

Went to play LP's the other day-no sound at all thru the preamp. All tubes are working on the Fosgate Signature phono pre-amp, but the two 6em7's in the Joule pre-amp are burned out.I find it odd that both tubes would burn out at the same time. CD playback is fine so CD player, speakers, amp are all ok, everything is connected properly. Any idea what would have made those two pre-amp tubes burn out silently? No pop or bang was ever heard-one day the phono chain was playing fine-next day zippo. Any help would be appreciated. I ordered two new tubes but am hoping nothing in the circuit was fried.