the QX serie is VERY GOOD, I´ve tested the 300.4, 900.1d (never gonna sell it), the QX90.2 and the QX180.2
For all amps a big plus is the sound processing, the quality is very good, the wrms´s are decently underrated.
Quality, stability and endurance are three words for the whole QX line. The titanium and tantrum line i´ve never had or tested (atleast no amps) but from what i´ve heart in ICE installs so far is that it has a very high quality of sound and the signal processing is REALLY adjustable at all levels.
A hint for PG products is to stay away from the new Octane-R line, these products (except the monoblocks) have very poor crossovers and a not so high soundquality. The monoblocks are a bit better but still have some problems playing "real low" and i´m thinking that they do not handle the power efficient.
QX serie is a overall good buy, think you will here it a lot from people on here