indigo,
The Anthem Pre1L is a very good preamp that can be had for $500, 6922 based so you can't swap tubes with the Mapletree headphone amp. However its relatively easy to modify and if you pick up the Ear+ as your first DIY project and decide to tackle the Anthem as the next DIY project I can give you some advice on it.
The biggest selling point of the Pre1L is that its a versatile preamp thats comfortable driving just about any power amp, soundwise it has plenty of extension to both extremes and doesn't sound very tube like at all despite having 4 tubes in the input stage. Those characteristics make it good for many applications and music styles if not as romatic sounding on acoustics as other preamps. In that sense its the same as a SHA-1, the SHA-1 being a much better headphone amp (the Anthem has a dedicated hybrid headphone stage but its opamp based) and the Anthem being a much better preamp. A big bonus here is that the Grado SR225 loves the Pre1L.
If you want cross-feed SFI's Line series preamps have Headroom's circuit integrated to the headphone output stage but its still not anything special driving Sennheiser HD600/580s or higher impedence headphones. Once again with Grados its a decent choice but I still think you should get a dedicated headphone amp to compliment it. The Ear+ is a great choice since Highwaystar and Nick Dangerous both like it and they know what they're talking about. A bonus with getting SFI's gear is you still have support through their former head designer, Chris Johnson, at Partsconnexion and he's really damned cool. The Line series are very good preamps but sound best when coupled with a sweet sounding tube amp, they can make a system sound too hi-fi otherwise.
A Melos Gold/Reference/Maestro used is also a very good choice based on sonics alone, there's been a lot of posts about it on AA and Audiogon as a preamp and it may be worth your while to read what people have to say. Unless you know someone who can trouble shoot and repair one if needed I don't suggest purchasing one, its too big of a gamble and recently many have died from damage during transit. However like the Anthem if you pick up a Melos there's support available should you decide to DIY modifications to it.
If you're willing to give up an integrated headphone jack in favor of a tape out (which you can use for your headphone amp) your options become much more interesting.
If you want something thats a bit more romantic sounding the Blue Circle BC21 is a damned great choice, I found myself infatuated with the unit's soundstage and decay and strongly considered purchasing the one I heard. However like you I enjoy electrics and found much to be desired there, its not an amp that can express a pissed off attitude. If thats something you can live with its a great choice provided that your power amp likes it (the BC21 isn't a great mate to every power amp, my CJ with 100k input impedence loved it).
The Rogue Magnum 66, if can find one at your price range, is the comprimise between SFI's type of sound - extension, linearity, fairly black backgrounds, dynamics, pinpoint soundstage - and Blue Circle's type of sound - sweetness, decay, texture on strings, depth of soundstage. The Rogue is closer to the BC21 in overall presentation but also gives you more dynamics and slam than the Blue Circle. Very well built with an outboard PS and great phono stage to boot, or you can ditch phono in favor of remote. In my and collegue's experience its comfortable with many power amps. Bonus for the Rogue is its 12ax7 and 12au7 based and finding quality tubes in that family is much easier than the 6922 family, plus you can swap/share tubes with the Mapletree.
If you want to go solid state consider taking the extra leap and go passive, however you'll have to be much more careful with system matching and actually care about impedences and cable lengths and the like. The Njoe Tjoeb (like you I'm an owner) has not served me well in my passive experiments for what thats worth.
If I were in your shoes I'd go with the Sonic Frontiers Line series, you're looking for the newer series with the black/silver face plate, if you can find it at your price range. Assuming you're interested in tube rolling you can exchange tubes for it with your Njoe Tjoeb, plus if you bought your CD player from Upscale Audio you have access to Kevin's cookie jar and his wealth of knowledge about rolling SFI gear. Otherwise I suggest auditioning the Rogue.