cadence has been in the business for a couple of years(their catalog says 17 years). they tend to stay above the mainstream car audio area's, and the stuff i have heard sounds very good-- i would put them in around the same cost/quality of focal, or audison..
i have heard very mixed reviews of the pdx series-- some say they sound great, some say they lack oomph on the low end.. i can't say i can notice it, just passing along what has been said over at diymobileaudio.com...
i have not heard anything bad about the hd series yet except the high price-- i was actually considering getting some of them for my own install-- but the fact that i want 2-4ch's and 2-1ch's adds up really quick..
audison-- same as the hd series, there really isn't much other then the price that can be bad about those amps..