I don't think a USB outboard device can be that much of an improvement over a PCI card. It's still being fed dirty 5V through the USB cable from the PC, and shares a common ground with the PC. Balanced output from a PCI or USB device would be an improvement, but the ultimate has to be S/PDIF out with an external DAC or processor. The one disadvantage of that approach is that you must do the volume control after the DAC, unless you don't mind giving up resolution by turning it down volume before the audio comes out of the PC.
One solution would be a switched attenuator between the DAC and crossover so you can keep the PC volume slider high under normal listening conditions, then kick the maximum volume up 10 or 20 dB when you need to get stupid. This of course should be done using a massive industrial control switch under a lockable plexiglass cover framed by black and yellow warning stripes and with a "Hearing Protection Required" safety sticker.
You could use a double-pole relay so the switch only switches DC and the wiring between the DAC and crossover is as short as possible.
power, you are correct. Some PCI cards are adding good capacitance to smooth out the dirty voltage. One advantage external has over PCI is that it can help eliminate EMI from the computer. I had a card once that picked up harddrive noise IF the IDE cable is too close. It's rare, but it can happen.