1) a headphone amp in general is designed to be connected to either line output of the source or the tape output of preamp.
2) RA1 amp is mainly designed to work with Grado headphones that do not have a high impedance and might not be a good driver for Senns. I had an experience auditioning a setup with Senn HD600/RA1 and I wasn't happy with the sound getting too bright and no bass extention.
3) You're right about connecting to the preamp tape output which probably will give you the best setup without reconnecting your headphone system.
2) RA1 amp is mainly designed to work with Grado headphones that do not have a high impedance and might not be a good driver for Senns. I had an experience auditioning a setup with Senn HD600/RA1 and I wasn't happy with the sound getting too bright and no bass extention.
3) You're right about connecting to the preamp tape output which probably will give you the best setup without reconnecting your headphone system.