I use a Brickwall in front of a BPT Pure Power Center (PPC). I've compared with the Brickwall in place with the BPT PPC , and without (BPT PPC direct to wall outlet).
The Brickwall filtering does no real degradation. Note that I use a Brickwall with the 20 amp surge unit.
I've also compared the Brickwall to some "filtering" power conditioners. Again, the Brickwall had no to minimal sonic effects, whereas the power conditioner had more sonic effects (particularly on dynamics).
IMHO, the filtering used in a Brickwall is pretty inert.
You can change how a Brickwall "sounds" by changing the AC plug or outlets. I snipped off the standard nickel plated AC plug and replaced it with a nice solid copper plug. That gave me more of the sonics I was seeking.