I'm using a Nad T742 receiver, the headphones are Sennheiser HD590, impedance is 130 ohm so driving them is not a problem. The source is Nad T533 DVD/CD player. Would I gain anything quality wise by driving the cans from an older Nad preamp model such as the 1240/1300/1600/1700 series ?
Probably the best way to gain a signifigant improvement is to go with a dedicated headphone amp. I've used the Musical Fidelity XCan with my HD600's and been very pleased.