Built-in headphone amps are usually an after thought - a simple op-amp circuit barely suitable for driving any kind of loudspeaker, even the tiny ones found in headphones. According to the Sennheiser website, your HD465s are 32 ohm, which makes them fairly hard to drive and might explain why your headphone/amp combination sounds poor. Grado headphones are excellent but are also 32 ohm so, if your amp is to blame, a set of SR-60s probably won't help. I'd recommend trying to find a dealer who will let you borrow a different pair of headphones and a separate headphone amp (you'll probably find the Pro-ject Head-Box fits into your £60 budget) and try each individually, and then buy whichever makes the most difference. Or, if you can find a dealer with a Kandy on display, try the same experiment in the shop.