I just went through this myself. I tested only the PDX 4.150 vs. the T400-4
That Alpine setup sounds better from the upper mids to the highs. I really like the midrange clarity of ICE powered amps. The highs are good but I've heard better for the price. The lower midbass is a bit lacking tough...
The amp is also said to be little nicer in the electrical system, according to the installers in Stereo King & Car Toys.
The RF setup owns it on the midbass. Again, just a characteristic of running a speaker directly from an output device (much higher damping factor). The mids where some of the best I've heard in a RF amp but different than the Alpine. Warmer, not as clear. Not unpleaseant tough...The highs could have been good, but I have a feeling the amp has some kind of built in EQ, so it was a bit harsh through my metal domes (Diamond). I hooked the T400-4 to some A/D/S sof dome tweets, reduced a bit the output with a 3 ohm resistor & got good results...now why would RF intentionally make an amp with tweaked HF response is beyond my understanding...
If I was running only one or my sub was to far to produce audible midbass I would go with the RF (I did). If I was running a separate Sub amp & my Sub could fill in for the midbass I would have gone with the Alpine.
I am not crazy about the upper highs or channel separation on either, my old Denon DCA 3500 owns them both in that respect (40 WPC, Quasi class A, Dual power supplies). All in all, both are great amps that will make a compact setup sound almost like one of the big guys...but you can do better for the $$$ if you have the space and the electric system to support it (I didn't but I am VERY happy with my stealthy 3 way system).
In an ideal world we could use the Class A to power the Tweets, the ICE to power the mids, the BD to power the midbass & the class D to power the lowest octaves...separate amps for each channel...but then the integration would be hellish... pick what sounds good to your ears & enjoy the music!!!
