This depends on your listening preferences. As shown above, tubes can work quite well with the HP-2. They're neutral, so they can go with everything, but they're also revealing, so don't hook them up to crap. I have listened to a Singlepower (Slam?) which was nice, but definitely missing the low bass impact and detail in the highs. I generally prefer solid-state equipment with my HP2 - the PS Audio GCHA was pretty decent and I've also listened to the Gilmore Dynahi and the Lavry DA10's amp - under loud meet conditions, unfortunately. The HP-2 works with most amplifiers except those intended for high-gain headphones (like the Sennheiser HD-600, at about 300 ohms vs. the Grado HP-2's 40 ohms).
I recently did a fairly exhaustive survey of headphone amps and concluded that, out of currently produced amps, the Channel Islands Audio VHP-2 ($399 USD + $179 VAC-1 power supply) would have been the most suitable for my budget and listening preferences. No extravagant claims, no gold-plated price. Reviews for the previous VHP-1 were favorable, and I expect the VHP-2 would be an improvement. HeadAmp's GS-1 would have been my second choice - I had some doubts about it since there haven't been many reviews of Justin's new modules yet. Also, the much higher price of the GS-1 ($749) and the wait time were factors against it. Justin didn't answer my email last week inquiring about the sonic differences between the GS-1 and GS-X, particularly for single-ended use.
I found the amp search was interesting because I hadn't looked in the amp section for a while. It was refreshing to see new products such as the revised Meier Audio amp lineup, Heed Audio's CanAmp, Trident Audio's LISA and also to recall works-in-progress (from when l was current) like the Beta22 and M3 in production with DIY makers. A lot of stuff looks good, but is unsuitable for one reason or another - too expensive, tubes, Europe-only distribution, or lack of sufficient reviews. The structure of the amp market was also interesting - Headroom is still the foremost manufacturer, while most of the other products are either one-offs from major manufacturers (e.g. Creek OBH-21SE, Naim Headline, etc.), extremely expensive boutique items (e.g. RudiStor's RP010B-MKII), or one-man operations like Dr. Xin, Justin, Gary, etc. Some of the most interesting amps that I came across were Lehmann's Black Cube, RudiStor's amps, and the Graham Slee Solo). There are also a lot more tube amps out there - I won't go into them here.
Some of the more senior members of head-fi might be able to help you regarding other amps. A number of them have heard numerous amps paired with the HP-2 either at meets or in their other listening experience. The out-of-production Melos SHA-1, Joe Grado's original amps, and the Mapletree amps also have a great reputation with Grados.
Personally, I made my decision a while ago with Norm's Headsave Classic. I recently took a look inside the case and was pleasantly surprised by the elegance and simplicity of the design - there were no extraneous parts anywhere, and the quality of the parts and connections were excellent. In my opinion, it's easy to get caught up in the marketing language and extravagant claims made by the various vendors. It happened to me again when I reviewed the state of headphone amps this week, but I'll be glad to be just listening when my amp is working again.
The Headsave Classic uses three Burr-Brown OPA627s, the same chip used in Headroom's Max amplifier. I find it to be lush, rich, musical, detailed, and accurate. There is no fancy power supply, only a 24V Elpac wall-wart... and yet, it provides as satisfying an experience as I've heard so far with far more expensive and elaborate equipment. I'm really looking forward to listening to it again with my new CD player. More on that later.