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

It is hard to say if you are going to have issues with placing IB1s close to the back wall.
I have IB2 and they are approximately 4 ft from the back wall, in a room that is approximately 18X18ft of unusual shape with no corners, so I have no issues of any kind with bass, ASC tube traps did almost nothing positive in my room (I had a chance to try).
A friend of mine has IB2s less than 2 ft from the back wall, in a room that is approximately 1/3 smaller than mine, of classical shape, but he managed to get quite good performance without huge peaks and dips by treating the room acoustically (pair of 13” tube traps, acoustic panels on side walls and ceiling).

Ryder,

I cannot say with certainty, but I would guess 17X15 or so, placed along the longer wall.
He is quite close to the speakers, around 9 feet, anything closer would not be good.
The acoustic treatment did wonders in his place, there is still a tad of boomines with some material, but nothing that is detrimental.
Seems to me that with these speakers it is more important that you are at the correct distance from the speakers, and at the correct height, than the distance of speakers from the back wall.

Ryder,

As a matter of a fact, my friend used to have the same speakers as you, PMC LB1 Signature, now replaced with IB2. The rest of system remained the same, with addition of room treatment.