P.S: I took a look at the descriptions and the manual for your Theta DAC/Preamp. The one point I’d highlight after looking at them is that in addition to changing the amp and the cables, by changing from RCA to XLR cables between the Theta and the amp you are now utilizing within the Theta (for each channel) two DAC chips, two volume control mechanisms, and two analog signal paths rather than just one of each of those things that were used previously.
In other words, while the signal applied to the RCA output connector for each channel is produced by the same DAC chip and subsequent analog signal path that provides the signal to one of the two signal pins on the XLR connector (probably pin 2), the other signal pin on the XLR connector (probably pin 3) is provided with a signal produced by a completely different (although supposedly identical) DAC chip and subsequent analog signal path. And that different DAC chip and different analog signal path within the Theta were not in use when you were connecting via RCAs.
You can see that in the block diagram on page 11 of the manual, as well as in the description at the Theta website:
http://www.thetadigital.com/downloads/Generation%20VIII%20Series%203.pdfhttp://www.thetadigital.com/generation_viii_series_3_dac_preamplifier_info.shtmlSo in addition to the possible causes of the issue that have been referred to previously, the Theta itself cannot be ruled out as a suspect at this point. And if changing the XLR cables to RCA cables + adapters resolves the problem, it would NOT necessarily mean that the XLR cables are to blame. In fact if that turns out to be the case I would expect it to be much more likely that the Theta is the root cause of the problem, rather than the cables, assuming there isn’t an outright miswire in one of the cables (which seems very unlikely given the good sonics you have reported).
Good luck. Regards,
-- Al