PMC IB1's close to rear wall, is this O.K.?

I don't have either of these speakers.

However, assuming that by "Proac rear porting issues" you mean too much bass boost between 80 and 100 Hz then this will be an improvement; the IB1 is much flatter in the overall bass than the Proacs, which should be a significant advantage when placed near walls or corners (where you get up to +6 db bass boost anyway).

If I recall bass goes like this Spendor - Harbeth - PMC - Proac in increasing order. ATC is actually regarded as bass lean by most people compared to "average" consumer speakers that tend to be a little bass heavy (bass sells).

Alun,

Yes indeed I was trying to give you an idea of bass strength...these are all good speakers including your existing speakers, selection being a matter of taste.