>ask yourself why would the designer not incorporate suitable mains filtering?
Judging by the interconnect thread, some people think it isn't so much designed as pulled out of a virgin's chuff 😉
Seriously – you could add a 'mains conditioner' but all it'll do is suppress typically non-periodic transient/switching noise*, and maybe some high frequency noise (tho' your amp supply ought to do that anyway). If that's what you're after then that ought to be fairly cheaply obtainable – I'd guess significantly more than 30 quid and you're being ripped off or into the realm of rapidly diminishing returns.
*actually, transient noise *is* high frequency by its nature, or more accurately, wide-band, but some of the spikiness may get through the amp psu – depends on how the amp's been designed and how spikey the spikiness is.
A UPS generates 50Hz ac from a dc supply – in doing that it'll generate some low level periodic noise (tho' not necessarily audible) – you'll get less supply glitchiness but you equally may get most of the way with a simple mains filter.
If you've got anything particularly noisy on the mains, it'd make as much sense to find whatever it is and deal with that (at a guess appliance motors, switches, dimmers).
>Amused at the thought of someone soldering caps across the mains
Cue 'what caps for house destruction or general messiness ?' thread 😉