Shortly after I bought my Bryston amps several years ago, I bought some AudioQuest power cords that were on sale at HCM Audio (regularly $120 each, on sale for $49). To be frank, I couldn't tell any difference between the AQ's and the stock power cords.
I have most of my system plugged into Monster Power HTS-2000 power conditioners, which provide a useful -- altho' not dramatic -- improvement in the overall sound quality (mainly reduced noise level).
Bryston amps are designed with professional use in mind, so their circuitry is pretty robust and does not seem to benefit a lot from after-market power cords and gizmos -- at least, that's my experience.