JCM2000 DSL 100--Dating and tubes

[jnl636]

I just picked up a JCM2000 DSL 100 in excellent condition. Is there anyway to find out the date of manufacture of this head? Also it still has the original tubes. From those of you experienced in Marshall amps what would you suggest as replacements for both the 12AX7's and the EL34's. I'm looking for the best tubes for that Marshall crunch.

[Enzo]

Serial number.