Marky P wrote:

As nothing is affordable on the open market, I'm planning on building
a switching box so that I can connect two 5.1 sources to the 5.1
analogue inputs on my amp. I was planning on using 3 double-pole
relays (BT47's) for switching the 6 channels. These would be
switchable by a front panel switch. Obviously, the unit would run
from a 12v power supply. How well would this work? Would the relays
cause sound quality issues? Any help would be appreciated.

It sounds just fine to me. No, the relays won't degrade the sound in any
way. Do make sure everything's nicely screened of course.

Graham