I'm not sure yet. When I find such an amp I'll let you know. My knowledge of the higher end amps, whether tube or solid-state, is woefully inadequate.
I found that the E9 was a big step up from any $100 amp you can buy (or build), pretty much no exception. It was similar in quality to a $300-$500 hybrid tube amp that I got a few years ago, the Dared MP5. Even with tube-rolling, the E9 traded blows easily with the MP5 to the point that I don't even use the MP5 any more.
I've heard some other more expensive amps like the LD MkIV, the Asgard, and HF EF5, none of them really stood heads and shoulders above the E9 so I haven't quite found a mind-blowing replacement yet. When I do, it'll become part of my gear repertoire. I hope to test out a Beta 22 amp soon. There's one being sold locally here and it's definitely making my wallet nervous.
You also need to ask what you consider to be mind-blowing. Are you looking for a huge boost in warmth? Are you just talking about sheer transparency and neutrality? Something with adds an unbelievably wide soundstage? Different people have different tastes and different ideas of what a "perfect" amp should sound like.
The E9 belongs to the "transparency" class of amps by my ear and its best attribute is its oozingly-liquid midrange. Its weaknesses IMO are a lack of body in the low end and it could be warmer -- but that's because I like warm sounding and bassy gear.
In the portable category I have the C&C X02 which I tend to use more than the E9. It's a portable but it packs a lot of punch, and it has a very warm, bassy and laidback signature which is just perfect. I prefer its sound to the Mustang, which is more than twice its price, although it's not by any reach a better amp than the Mustang, as the Mustang is clearly superior in the midrange comapred to the X02. I have yet to find a mind-blowing amp to replace the X02 as well, although the X02 pretty much blew away my older portables like the E7.
Edited by Mochan - 8/26/11 at 8:34pm