Hello Hawklord!
In my (admittedly rather limited) experience, high quality headphones benefit more from a good headphone amp than lower quality ones do.
I think that Sennheiser HD600s are
really good headphones, and they're actually pretty good value too. You can buy a pair from ASK Eelectronics (click
here) for £140 - you might get a better price if you shop around.
If that's a little too much, then you could try the almost-as-good Sennheiser HD580s. (Maybe give Harrow Audio a call on 0800 174 219 and see if they can quote you a good price).
Headphone amp-wise, I suggest you audition the Earmax (if you do buy Sennheiser 580s or 600s then a standard Earmax rather than Earmax Pro might be okay) and also the Graham Slee Projects "Solo" (which one reviewer reckoned outperformed a headphone amp costing £1800). The Sugden Headmaster is apparently
very good, but at £600 is also pricey. As mentioned above, though, it doubles up as a very reasonable stereo pre-amp, so if you use a separate pre-amp anyway, it could be a good option.
Earmax and Solo are both somewhere around the £350-£400 mark, so perhaps a bit out of your budget range. If you want something a bit cheaper then there are various possibilities from the likes of Rega, Creek, Musical Fidelity, and so on.
If you can afford something like the Earmax or Solo or Headmaster I would get it - they're quite a step up from most cheaper models - but even a cheaper amp may sound much better than the output from many headphone sockets. You have to remember that manufacturers rarely devote any significant time or money to boosting the peformance of the headphone socket, because they think no one will use it - so any money they spend on it is money that their rivals can deduct from the price of competing products without affecting the performance most customers get out of it.
That said, there's still going to be a
big gap in performance between the likes of your DT231s and the higher-end Sennheiser 'phones. (The DT231 Galactics are good value at around ~£40, but much better is available if you go up the price scale).
Another possibility you might consider is investing in a second hand system from Stax. At first glance Stax systems look very expensive, but when you compare the price of the system as a whole with the price of conventional 'phones plus headphone amp, it's not so bad.