New tubes or new preamp?

I have an ARC LS15, and am considering either a set of NOS Amperex tubes, or selling it and getting a new preamp, such as an ARC LS27, or a Modwright LS 38.5.
As I'm unable to compare , what would you advise?.Will NOS tubes give me a preamp that compares with todays current ones, or is it a waste of money?
Buy 4 tubes or a preamp that costs more than double the price of your current preamp is an amusing question. I think NOS tubes have become extremely overrated. If you want your LS-15 to sound its best you should send it back to ARC for a complete checkup and fitted with tubes selected by ARC. If you want an upgrade buy the LS-27.

I cannot comment on Modwright products.
I got a set of tubes from ARC and they were so microphonjc I Sent them back. Really want to know if spending money on an older per is worth it versus a new one.
There's more to a preamp than just the tubes. For me, the preamp is the most important piece in the system. (Providing you don't mess up matching your other components.) Get the LS-27. Its a big upgrade.