There is interaction between the channels when you bias the amp. As you bring one channel up, the other drops. You need to go back and forth until they are alike. If the left channel biases up to the proper voltage, and is higher than the right channel which seems to top out at 1.50 volts, then back off of the left channel bias. The right channel should come up a bit. You can probably get an even bias setting between the two channels, maybe around 1.53 volts.
I would try the following:
1 ) Turn off amp
2 ) Rotate the bias pots in both directions until full stop. This can help clean the pots. If you have some Cramolin or other quality potentiometer lubricant/cleaner, try that as well.
3 ) Rotate post fully counter clockwise.
4 ) Power amp on and let stabilize for 15 or 20 minutes.
5 ) Note the bias voltage for each channel.
6 ) Adjust the bias for the channel with the lowest initial voltage first.
7 ) Adjust the other channel to proper bias voltage.
8 ) Go back and forth between the channels to get a balance close to the desired bias voltage.
Once the two channel are balanced, operate that way for several hours/days, keeping an eye on the bias for both channels. It's possible that the weaker tube may bounce back after significant use.
I've run the U70 biased from 1.2 vdc to 2.0 vdc with no trouble. The bias voltage isn't the absolute setting to consider. The whole idea is to bias the tube to a certain level of its maximum plate dissipation. The rule of thumb is around 70%. The JJ 6CA7 has a 25 W plate dissipation, so 70% would be around 17.5 watts at idle. This translates to a bias current of about 42 milliamps per tube, or 84 milliamps for the pair based on 415 vdc on the plates of the tube. If you have accurate 15.6 bias resistors in the amp, then the bias voltage to set the tubes at 84 milliamps would be 1.32 vdc. See if you can get the bias voltage at least to that value and run for a while. Running the tubes a bit on the low side won't hurt and will give longer tube life. If this works out, you can go more towards the 1.56 if you like.
Hope this helps,