I've had the Retro power amp for a while now and I am really getting a great amp sound and feel from it. For comparison purposes, I have tried it with:
Atomic powered cab - with internal amp off obviously
Line 6 Vetta extension cab - 2x12 mono/stereo
Zilla studio pro custom cab - 2x12 mono/stereo, open/closed back options
I prefer the atomic using the Retro amp compared to using it with its own internal amp and, whilst it's pretty good for cleans, I've never really jelled with distortion using the atomic and that hasn't changed. Its better now with the Retro, but still not satisfying for me.
The Line 6 cab, surprisingly sounds pretty good. It needs a bit of tweaking out on the mids, otherwise it sounds harsh. Tweaked it sounds pretty good. The speakers are Celestion and are designed to be pretty flat - it's the extension cab for the Vetta combo, so it's designed to be used with amp models.
The Zilla cab gives the best results for me though. It has Celestion vintage speakers, a V30 and a G12H. Like the Line 6 cab, it is wired in mono/stereo at 4/8 ohms respectively. The best results are found in open back format compared to closed back, although I am a classic rock rather than a really heavy guy.
I set the Axe II with amp and cab sims on and mic sims off. After some tweaking, I now have it sounding like a range of great amps. I've played the USA models side by side with my Mesa Mk V and the Ultra/Retro/Zilla combo wins. I have NEVER had that before. The valve amp has always won and it is very exciting to finally reach the point where that is reversed. I'm trying to make my comments fairly restrained and balanced, but it is quite hard to do so as this is really a big break through for me.
Using a conventional speaker cab, most of the tweaking necessary is actually with the speaker sim rather than the amp sim. The Axe amps sound great pretty much on stock settings once the speaker sim is tweaked right. I'm setting it with a bit of low/high cut, Motor Drive pretty much off, Air at around mid 20% and no mics. I also tweak the mids out a bit on the Mid Resonance control on the amp speaker tab and sometimes tweak the amp Xformer match a touch. That's pretty much it, I don't touch much else other than to add a bit of the new amp compression.
Anyway, that's my reflections on the Retro for what it's worth. I've learned quite a bit getting there, especially the importance of the speaker settings in conjunction with the settings in the amp speaker tab. When the amp/speaker settings are right, the amps don't actually need much tweaking at all to sound great. I'm still finding it hard to believe that I finally have something that sounds and feels just as good as my amps, but with so much more versatility. I'm probably only at a point that many of the people on here are at, but it is bloody great to finally get there.
If you are after a real amp/cab solution, I would seriously recommend at least considering the Retro power amp.
