I have 650s, 595s and 555s. I really want a headphone amp to give my integrated Cambridge Azur amp a kick up the arse. But I am soooo confused by all the threads on here. Do I need a Gilmore Lite or a Headsave Classic (not that they're made anymore)? I can spend up to about £300 ($550ish) although I'd really rather not spend that much. Please help. And, if you can, avoid jargon because I have a small brain that is already full.
Do you have any hi-fi stores around where you live that stock ANY headphone amplifiers? Why don't you take your phone down to the shop and have a listen to see if ANY amp changes the sound (the way you like it to) of your headphones.
I have not heard the two amps you listed but I had consider buying a GL, they should drive your Sennheiser well, and Grado's if you eventually come to the 'Dark side'.
As you are in the UK you could try either the WNA (White Noise Amp) available in kit form if you are handy with a soldering iron. I think it's around £150 kit or about £200 pre-built but you'll need to check current prices.
Another good amp for Senns is the pre-built ANT Amber which should be within your proce range. Alex does do a trial period so you can make sure you are happy with it, again check current prices.
Both of these have plenty of threads detailing the amps here so a quick serach will pay dividends.
Where in the UK are you, there may be a head-fi member nearby who could let you try on of their amps.