If the DC voltages are correct, this isn't likely to be a tube issue. This could be an EQ issue, and missing one of the jumpers on one of the central terminal strips between the EF86 sockets would leave one channel far, far louder than the other and without any EQ. The challenge with something like this is that sometimes the louder channel is the one with the issue, so you really have to look at them both and check over your wiring well.
If you need more specific assistance in determining which channel is faulty, having a phone/tablet/computer with an 1/8" headphone jack will let you plug a TRS to RCA cable from that source into the Eros, then you can play a 60Hz tone into the input jacks and measure both the AC voltage at the input jacks and the AC voltage at the output jacks, and we can let you know which channel is working properly.
Also, just posting build photos sometimes is helpful. I repaired an Eros with a very similar complaint not that long ago, and one of the big gold cathode bypass caps up front had been installed on the wrong terminal on one channel, and admittedly I felt a bit guilty that I hadn't asked for build photos before we went through the whole repair process.
-PB
