Sennheiser and Grado are like polar opposites sound wise, make sure that's what you want before you buy. Otherwise you might just want to try a higher end Grado.
Vintage amps can be quite good for headphones, my Sherwood S-7210A was able to best my old Burson 160DS for not even 1/10th the price. The only real issues you come in contact with would be impedance mismatches. That only really would come up with low-impedance phones like your Grados though, if those sound fine then I don't think you'll have any worries. You can usually tell if you're having impedance problems because the bass will sound 'un-natural', if you had that problem you'd pick up on it rather quick.
