Not sure I can explain phase linearity on caps in a couple of words, wikipedia might have an article on that (or some audio site) might do a better job.
5.5 uF might just do the trick, I'm quite sure you do not need 44 uF
(unless you are using an amp with a very low input resistance and did cut our the 47k resistor at the output of the DAC),
In short; Caps affect the phase difference between current and voltage by nature, the larger a cap gets the lower the frequency roll of starts. If you use a 'borderline' sized cap of which the roll of value is close to the audible range you may hear/measure the full low freq range yet there is less power available, and subjectively you'll hear less ' slam'/'push' in the bass registers.
I'm not sure whether changing your pot is the best approach to get away with a lower sized cap, the pot influences input impedance for the DAC and the cabling so that would work, but only if the amps input resistance is not fixed(see later) BUT the first amplification stage may be affected when you use too high of a value (they did not use the 50k Pot just for fun, I hope).
IMHO the better method (and more linear too) is to connect a resistor of appropriate value between the input 'plus' and 'minus' of your amp (in your case 47k) as the changing resistance of the pot may influence sound quality.
Most tube amps have an input resistance of about 100k, while transistor amps typically use 47k.
Pat, I get the feeling that you are also noticing the effects of input-output resistance mismatch. In some cases a buffer amp can sort that out or indeed the good old transformer can help out and provide galvanic decoupling at the same time (BTW I love these chunky parts, though high quality parts are very pricey).
The resulting sound quality of either option can imho not be generalized to 'use a tranny for more bass' as the performance of either option will be depending on the efficiency of the resulting match between all equipment thus linked and the individual quality of the parts used.
perhaps as a tip; I always put two trannys at a 90' angle to each other as the magnetic fields may well influence each other