Yeah I couldn't get on with the K550 and my main problem was the fiddly, loose fit that destroyed what little bass it had. Some people are ok with very weak bass and thus there are fans of the K550, but not me and apparently I'm not allone. It's not the headphones on't problem though, the highs were a bit annoying to me as well. I should point out that it bugs me even more when the bass overwhelms the midrange. I've recently recommended the super cut price WeSC Chambers (£250 down to £50 in the UK) to people because I thought the sound was really good, but most of the time I try to EQ the bass down a few dB.
I am guessing that the K550s are your problem, rather than your amp. Any amp that's remotely well recieved is not going to be the biggest contributor of providing a sound that you don't like. The only slight thing I will mention here is that I found the K550 mostly sounded better on my phone rather than a dedicatd head amp and this was because my phone, at the time, pushed the bass quite a bit. This was the main reason why I fell for the K550 after testing it in a noisy shop. I think I got lucky with the fit, the isolation must have worked quite well and I was probably pressing the K550s on to my head at the time. I found that holding the K550's cups hard into your head makes the seal much better and improves the sound a lot, although it doesn't fix the annoying treble.
Anyway, I have found a few headphones that I really like that have nice, powerful bass but also sound good everywhere else too. Here they are in order of bass (most powerful first)
Denon D7000, Fostex TH900, Fostex TH600, Denon D2000, SoundMAGIC HP200, Hifiman HE-400, Hifiman HE-500, Sennheiser HD650, SoundMAGIC HP100, Sennheiser HD600
Now the HP100 is interesting because it's generally pretty neutral sounding (like the HD600), it's near the end because the bass isn't in your face in any way, but it's very enjoyable and oddly (nicely) smooth. Even the two models at the end of this list are miles and miles away from the bass-shy K550, they don't have that hollow feling that the K550 does. The HP100 is one of the only ones in the list that isolates really well. I found that generally good isolation kills soundstage, but the HP100 isn't too bad in that regard either. A good compramise if you want great soundstage and bass are the old Denons, I haven't tried the new Denons yet so I can't confirm if they are any good. The new Fostex models (in the list) use the same design as the old Denons (Foster Electronics made) and they're great, but they are very expensive!!