First off a bit of terminology that has been bothering me, USB is a bidirectional BUS, even with digital audio it is not a one way street, most of the data goes from computer to DAC, but not all. There are two sets of official terms to describe the ends:
host/device and upstream/downstream. The computer is a host, the DAC is a device, the REGEN is a hub, which has upstream and downstream ports, the upstream points towards the host, the downstream points towards the device. I will use these to refer to connections on boxes. I will reserve the terms transmitter and receiver when dealing with actual data flow in and out of a port.
For example when talking about connections say downstream port of the REGEN is connected to the DAC. But when talking about signal integrity (SI) it is appropriate to talk about the receiver in the DAC PHY, since that is the part that is susceptible to SI. The transmitter in the DAC PHY is not active very much and has little impact on noise in the DAC. OTOH the transmitter in the REGEN's downstream PHY IS important to the SI received by the DAC PHY.
Since the receiver in the REGEN's upstream PHY is susceptible to SI just like every other USB PHY in existence the quality of the signal it receives is still important. So using a good cable and good host PHY are still important. Whether a PPA USB board in the computer feeding a good cable without a REGEN is going to be as good as a regular USB port and good cable and REGEN at DAC, who knows, there are so many variables it's impossible to make a general statement on that. BUT there is a good probability that the PPA AND the REGEN will sound better than either.
So if you already have a specialty USB card (Paul Pang, SOtM, JCAT, etc.) in your computer, don't throw it away when you get a REGEN, the combo is most likely going to sound significantly better. You may find that just using the REGEN without the upgraded SBB host card is not much worse than both, in which case you might find it a good idea to use the card on another computer that has been begging for an upgrade.
Remember that SI consists of rise/fall time, noise, and jitter. The jitter in the SIGNAL is determined by the transmitter PHY, which can be significantly influenced by the clock IT gets and the noise on its PG planes. USUALLY noise is low on the signal as it exits the PHY. The cable (and connectors) cause an increase in raise/fall times, added noise (EMI and crosstalk from power and ground wires) and decreased amplitude of the signal. Any decent receiver will have an automatic gain control (AGC) which compensates for this effect, but that raises the noise on the signal, so I'm lumping the amplitude decrease into noise. The cable by itself rarely adds jitter to the signal, BUT the increased rise/fall times and extra noise cause the received data to have increased jitter in the PHY. This is one of the big issues that all that extra processing is designed to deal with.
When that XMOS chip is dealing with data from the host it is generating its own noise on the PG planes, part of the noise will be from the PHY and part from the MAC. The PHY part can change due to SI of the USB signal, the rest cannot, it stays the same. If separate ground planes and separate power supplies are not used, that noise can directly affect the DAC chip(s) and the master clock oscillator. As was posted from a previous post of mine, even if separate supplies and planes are used with isolation between them, the effects of this noise still winds up at the DAC chip and clock.
The part of this noise not from the PHY is always there, it doesn't matter what your cable is, what USB card you have, whether you have a REGEN etc, it's still there. All that stuff is keeping the PHY part from ADDING extra noise that makes things worse.
The technically correct solution is to figure out how to prevent this noise from crossing the barrier and getting into the DAC chip and clock, unfortunately this is really tough and nobody has yet to completely figured out how to do this. Thus every DAC ever built will have some level of susceptibility to external influences, some more some less.
By the way, just because a DAC is not very sensitive to external influences does not mean it sounds really good, there are lots of ways to decrease the sensitivity that muck up the sound. The trick is decrease the sensitivity AND do it in such a way that doesn't decrease SQ.
So everybody that is tweaking their computers, using different cables, a REGEN etc are all still hearing the effects of the non-PHY noise. If we can figure out how to get rid of THAT, WOW, you won't know what hit you.