The Quad 606 features the current dumping circuit which means as the internal components age, this amp will self adjust to continue to produce quality sound. Internally, it features a small Class A amp cancels any distortion in the "current dumper" amp. Regardless my overly simplistic description, the 606 always sounds musical, and is open to a wide range of speaker loads should you ever change from your JBL's. Also, the Quad leans to the mellow side of neutral, something which might match well with the JBL. 606 is just a great amp, though not as widely appreciated in the U.S. just because it's not a U.S. manufacturer. The Quad current dumping circuitry was/is one of the few true innovations in solid state amps. Most amp designers simply tweak power supply, play with bias, etc. Peter Walker was an innovator and that amp will prove it to you.