To answer your question about the XLR connectors, I wouldn't worry about it unless you have fully balanced equipment. Stay with rca.
To answer your question about cables, I don't think its a smart move. Your DH75 is overkill for your equipment. Not only that, just setting a $300 budget is foolish. Go by sound quality, not price. Look at your system in context. Do you really think a $300 digital IC is a better move than upgrading an active component that will have a much bigger effect on sound quality? You're only going to get so much out of your NAD. I would start there. Your amp allows you to jump the amp and preamp sections externally using rca jumpers. I say put the money towards a better preamp and plug it directly into the amp section of you NAD. I don't care how expensive the cables are, theres no way they will make a bigger difference than upgrading an active component.
To answer your question about cables, I don't think its a smart move. Your DH75 is overkill for your equipment. Not only that, just setting a $300 budget is foolish. Go by sound quality, not price. Look at your system in context. Do you really think a $300 digital IC is a better move than upgrading an active component that will have a much bigger effect on sound quality? You're only going to get so much out of your NAD. I would start there. Your amp allows you to jump the amp and preamp sections externally using rca jumpers. I say put the money towards a better preamp and plug it directly into the amp section of you NAD. I don't care how expensive the cables are, theres no way they will make a bigger difference than upgrading an active component.