pass labs xa100.5 or pilinius sa 103

Don't know about the Pass x350.5 but I would take Spectron (with the upgrades) over the XA100.5 (I own both), but again, it's subjective, all amps mentioned int his thread are very good.

Thesoundhouse.

If you want to compare Spectron with Pass Lab amp - and I love both (and I love Plinius too) but preferred Spectron - then compare them in "apple to apple" manner.

Pass Lab X350.5 or 200.5 or 100.5 cost much more then stock $3.9k Spectron. Compare Spectron stereo or better monoblocks or better monoblocks with upgardes until the prices will be more or less comprable. Howeveerr, still Spectron will be cheaper but its Spectron's business to price their product competitively not customer's. I wonder also what speaker you use and what type of music you prefer but its secondary to fair (more or less) equipment comparison.

I have the C4s as well..Pass without a doubt.Much better match than the Plinius,IMO.You may want to addresd new speaker cables as well