I think it'd be a push to get it much darker (alright, I sanded it a bit too fine before the deep base coat which means it didn't take quite as much colour as it would if I'd stopped at 240-grade ... but I don't think there'd be much difference in it).
I think if you wanted a REALLY dark red you might have to mix it with a walnut-brown or something similar ... DB would be the one provide guidance on that ... I was after initially after a really dark red myself but after seeing the almost "antique violin" finish that I've ended up with I'm really quite happy with it