It seems like all the KEF and PSB speakers I've seen have excellent frequency response measurements.
I've only auditioned the PSB Imagine T and the KEF 201/2 speakers.
The KEF 201/2s sounded much better - clearer, more detail and resoluion in the midrange and trebles.
Of course, the KEF 201/2s are retailed at $3K each, and the PSB Imagine T is $1K each.
If you feel the KEF center speaker is much better than the PSB, I would get the KEF center speaker plus the KEF front L/R speakers.