When did you start having this issue? Have you definitely isolated it to your amp? Seems somewhat more likely perhaps a tube failing in your preamp despite it being relatively new. Try just reversing the connections between you preamp and amp and see if the volume issue switches channels. Good luck!
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Also check out this page for more info on your amp:
This passage seems possibly relevant:
3. OUTPUT TRANSISTOR BIASING: The SOLITAIRE has 4 pairs of output transistors and when 'biasing' the amplifier it is wise to check the biasing of ALL the output transistors to ensure that one or more of the devices are not conducting more current than the others. This is especially important when replacing a power transistor. Also check the different biasing currents between the NPN and PNP devices. If there is a large discrepancy, replace the 'driver transistor' (also mounted to the heat sink).