I would suggest Rega Ear or Project Headbox SE - advantage og the headbox is that there are 3 inputs and pre out with the bonus of 2 headphone sockets each with volume control.
On a personal note I'm selling one of the above as I can't justify having 2 headphone amps. I do a lot of headphone listening and have used many amps - from the non SE headbox to a Graham Slee and McCormack MID.
one of those cheapy Little dot II's on ebay in my book represents excellent value. I stuck in a best offer of about £60 and won. Great sound for the money. Runs a tad hot but don't let that put you off. the stock valves area little crappy, so well worth the modest outlay to upgrade them.
These phones will be fine with any of the amps mentioned. I've not heard the little dot - only positive opinion from others - but a valve headphone amp for less than the 2nd hand price of either of the 2 I've suggested can't be bad!
Did some asking around - apparently the Rega ear was designed for Senn HD 580 / 600 (?) headphones. it provides a lot of gain which is apparently good for the Senns but can make "forward sounding" headphones like some Grados sound too harsh.
I have a Little dot 2++ and a G&W TW J1 (both from China).
The J1 wins hands down.
I've used them both with Grado SR-125's and Sennheiser HD-650's.
The Little dot suits the Grado's better - warmer tube sound balanced by the more in your face Grado sound.
The J1 makes quite a fine Preamp too (if you one have one source).
For £60 on ebay + £20 postage to the UK you cannot go wrong.
A 'hifi pal' of mine into serious Hifi briefly auditioned the J1 against a Graham Slee on Sennheiser HD-650's. His words were
"yesterday I *briefly* re-auditioned the J1 - this time against the graham slee. (well I was walking by ...) was not a serious audition but based on 30 seconds, minimal difference I'd say !"
My 'hifi pal' is working in Shanghai at the moment...
They have whole shops that sell nothing but H/phones and H/phone amps.