I have not personally had this problem but I would guess that this is not normal. Looking at the schematic there is only a few components in the lead mode which are not active in rhythm mode.
The first thing you can do is replace V1 with a known good 12AX7 (aka ECC83 or 7025). If the volume does not go up with a known good tube, next thing to check would be the cathode cap on V1. If it's shorted lead mode would not work. If it's otherwise bad, gain would be lower. However, that should make the tone less distorted but not necessarily less loud.
Next, if the coupling cap off V1 (it's C5 on the schematic) is leaky it would throw off the operating point on the second stage. In the rhythm mode you would not notice it since it's out of the circuit in rhythm mode. If it is bad it must be replaced.
Lastly, the optocoupler LDR2 could be bad and not correctly close the circuit in lead mode. Ideally when in lead mode it should have a resistance of zero. A small resistance (say up to 1k) would not be a problem but if it is much more, it could negatively affect volume and tone.
The only other component which is added for the lead mode is a 22k resistor to ground in parallel to the master volume. It's purpose is to compensate for the higher gain and level of the lead mode. I would check that it is really a 22k (and not a 2.2k). This resistor could be increased to 27k or 33k to provide a better balance but I would pursue that as a last tweaking measure if everything else is working.
Schematic is here:
https://schematicheaven.net/boogieamps/boogie_50cal.pdf.
If you are not familiar with tube amps, best to take it to a qualified technician, apart from the V1 tube change. I would do that first, and I would not be surprised if the amp then works correctly.
Good luck,
Stephan
