I use the earlier model - GSP Solo Mk II - with HD600s, and the result is very lovely indeed. I haven't, I must admit, personally auditioned any of the more expensive headphone amps for comparison. My "Solo" was bought a long while ago, while the business was still being built up, and when they were selling directly rather than via dealers, so at that time it was significantly cheaper than it is now, and comfortably outperformed everything else in its price bracket; now-price-comparable devices like the Earmax were then comparatively much more expensive.
The "Solo" has two sets of inputs, so you can connect two sources directly, or you can use some other device for switching - for example, hook the headphone amp up to the tape outputs of a pre-amplifier.
(I've found Graham Slee himself to be very approachable, btw, so if you have any significant issues, ask him).
If you're currently connecting your phones direct to a source's phono outputs, don't.
Whether you want to use your DVD player or your processor to produce a stereo signal is going to depend on the comparative quality of the two devices - it's not a clear-cut thing. For instance, I find my Bryston SP1.7 sounds as good as my Arcam DV27 for plain CDs, but for HDCDs the DV27 sounds better, because the SP1.7 dosn't understand HDCD and treats them as 16-bit only.