Being a NEWBIE here goes

Using my profile as a guide to what I currently own, what piece of equipment is a practical upgrade?

Profile: amp phase linear 400
preamp sae mark IX
marantz cc65se
magnaplanar 1.5qr
rel storm III subwoofer
monster interconnects

Note current amp is about 30 years old which I purchased in 1973.
I really enjoy the Magnaplanars with the Rell subwoofer. I would like to possibly upgrade to 3.6r Magnaplanars but the age of the amp has me worried that if I commit that kind of $$$ to speaker upgrades and the amp fails I'm out of luck listening to anything.

Impact wise I think the speaker upgrade will give me the most bang for the buck or am I kidding myself? Anyone have any opinion on what kind of impact the amplifier change would have and if so, what amp in the $2500 range would you suggest?