Unreceivedogma, it appears that your preamp provides two RCA outputs. "Output 1," at least in stock form, has an EXTREMELY high output impedance of 100K, that was apparently suitable for use with a matching Beard power amp, but very definitely would not be suitable for driving long cables or a majority of other amps. "Output 2" has a specified output impedance of 600 ohms, although I wouldn't be surprised if that number rose to considerably higher values at deep bass frequencies.
Also, it appears that your sub has an external crossover/amplifier, with a fairly low input impedance of 20K. I don't know what the input impedance of your amps may be, but I suspect it is relatively high.
Before providing any suggestions, my questions at this point are:
1)How are you connecting the OTL amps and the sub's crossover/amplifier to the preamp? If one is connected to "Output 1" of the preamp and the other to "Output 2," which is connected to which?
2)By any chance do you know if the capacitor upgrade that was performed on the preamp included the output coupling capacitors?
3)And if so, do you know what the values of the new output coupling capacitors are, i.e., the number of uF (microFarads)?
4)If you know the input impedance of the amps, that would also be relevant information.
Also, a minor correction for the record, if I may: The musician you referred to is Al Kooper, not "Cooper." As you may be aware, also, prior to his BS&T days he was a member of the Blues Project, a great group IMO!
Regards,
-- Al