Generally, dedicated (and well designed) headphones amplifiers will give you much better impedance and voltage/current/gain to drive headphones than the output from amplifiers.
If you are interested in reading about headphone amplifiers in general I'll point you to NwAvGuy's blog, he's a fountain of knowledge when it comes to headphone engineering and has got some great articles about headphone amplifiers, how they measure, impedance matching, snake oil in the industry, etc. http://nwavguy.blogspot.co.uk/
He has actually designed a headphone amplifier that is making waves in the headphone community, designed to give the best performance and measurements possible at the low cost. His design is completely non profit, but I think there are some distributers in the UK. It's called the objective 2 (o2) headphone amplifier, and is well worth looking into if you are considering purchasing an amp.
The second recommendation I would give you (keeping an open mind) would be a chinese made clone of the Lehmann Black Cube Linear called the Matrix M-Stage. It's something like a quater of the cost of the original and offers pretty much the same level of performance, with a pretty decent build quality. Again I think there were some distributers in the UK.
Saying all this, I think the grados are low impedance and fairly easy to drive, so it might not be worth it. Hope this has been helpful and good luck!