Have a look at the Pro-Ject Headbox S2 @ £125. It has analogue in and a loop out connection so can stay connected in-line between the Lenco and your Edifiers.
Pro-Ject knows a thing or two about good Hi-Fi gear without spending daft money. So you get something that's both built well and performs well for the price. There will be cheaper headphone amps out there, tonnes of them, but mostly for home DJs where audio fidelity isn't the main criteria. If that's you then have a look at the sort of retailers that sell guitars, drums and recording gear. Beware though, their starting point might be different to yours and how the things connect could prove tricky. For example, the Rolls Bellari is around £160 and has a loop-thru output. But you need to use the balanced XLR inputs for loop through to function. You don't see that in the web pages unless you really dig around a bit.
The IFI ZEN Can Air is a decent shout at £99 if you don't mind swapping some audio cables as and when.
Unless you're looking at secondhand, and if you want it to stay connected inline, then the Headbox S2 is where I'd be. If not, then the IFI.