Based on the suggestion of Vaughan and Trevor at WGS, I installed the ET65/Retro 30 speaker mix in my 2X12 Sour Mash cab today. Fired up the '67 Bassman and holy smokes, this cab really came alive. I think "clarity" is a great word to describe the tone of mixing these two speakers. There's a great warm but tight low end, really nice mids that will ride just above the mix in a band setting and the highs are nice and clear without being piercing. I also like how the Retro 30 has an SPL that is 2db higher than the ET65. It keeps the lows of the ET65 in check as the Retro 30 is just a hair dominant...not in a bad way at all. It's a very nice mix of the two and they really compliment each other.
I'm not a high gain player but play mostly American style, high octane rock-n-roll. This is a really great match for the type of music we're playing. I can only imagine after a few shows, rehearsals and recording sessions these speakers will loosen up a bit and be even sweeter. The guys at WGS really seem to know their mixes and how to choose a speaker to fit a particular amp and style of music.
On a side note: I've been running my Bassman with an 8ohm cab (loaded with a Private Jack and Classic Lead 80) and was really surprised by how much more headroom and bass response I have now that it's paired up with the new speakers loaded to make it 4ohm. I had to reset a few settings but man, jumping the bass and normal channel on this and driving it with my old mid-80's Tubescreamer get's me exactly where I want to be.