I have a GS-1 and I have tried it with the HD650. However, my comparison is probably biased by the fact that I am so used to hear them together with a MPX3. The GS-1 and 650 are a good fit, but the MPX3 more so, but it is only just in the bass region you find any differences worth mentioning: I listen mostly to orchestral classical music and the decay of bass notes in large spaces is then of importance to me. The MPX3 just manages that a bit better, holding on to the reverb for just that little appreciated extra long time and with more ease and air. But really, in absolute terms, the GS-1 has a very good extended and controlled bass. However, during the burn-in period, the GS-1 struggled in that department with the 650s, but I seem to be the only GS-1 owner here to have reported that. Most of the reports have talked about an exaggerated bass during that period.
As to aftermarket cables. Well, I have only tried the Cardas, and it is supposed to have a quite gentle influence on the phones - but I have mixed impressions. If you ask me, the stock cable appears to have a very good synergy with the 650. It is also important to keep the nature of the 650 in mind. The metaphor "sitting in the middle of the concert hall", in stead of the front row, is a good one IMO. The sound seems to get a lot of freedom to develop, and spacious details the possibility to find their way to you in a “natural” manner, not forced. An aftermarket cable will most probably alter the balance of the cans. For good or worse, that’s up to you to judge. I’m with gpalmer here: Try them with the stock cable and see to it that they get a proper burn-in first.