I know this probably isn't the best website for audiophiles, but I want to get as many opinions as I can. I am currently using the HD 598's from Sennheiser with the normal on board audio from the Z87 Sabertooth motherboard. I'm thinking about upgrading to the HD 650's, and what I need to know is whether I would benefit from using the Amp with my 598's, and if I would need it for the HD 650's.