There is no difference in clarity/sound quality between XLR-XLR and XLR-1/4" TRS cable
It's a still little bit confusing. I also read that only 2 first channels can be used so that preamp is bypassed, but maeby i underestood it wrong.
This means that if you press in the instrument button for channels one and/or two you can plug in a guitar or bass on the front panel. It's a DI!
However, if you wanted to patch in an outboard preamp, you wouldn't use the instrument in on the front, you would use a TRS 1/4" from your pre into the XLR/TRS combo jack on the back.
If you were looking to add another device to your 2626 via lightpipe, something with preamps, but that can also serve as a straight AD converter... get another 2626
Surely someone can chime in and offer this fellow some other suggestions?