I tried a GK RB400 at a gig at Leonards in London once and it was love at first listen. I was playing a pair of 2x10"'s and an Eden WT800 had at the time. So I sold the lot and got a pair of RB700 1x15" combos instead and have NEVER regretted it. I've compared and contrasted the amps against Trace, Ampeg, Markbass, Epifani and the GK's won through on evenness, frequency response and clarity. The controls are centred at useful frequncies, the boost control literally dials in extra growl, its bloody loud and very noise free. I can also play a variety of basses through the combos and they all sound good. I could never do that with either the Eden or the SWR heads I owned previously.
I really can't fault the amps sonically although the wheels on the combos can be fragile over rough surfaces.