Tube recommendations for ARC LS-2

What do you mean by improve the vocals? Do you want more 3-dimensionality, presence out into the room or harmonic richness? You can try all sorts of different tubes here but these qualities are simply not going to be found in the LS2....like squeezing water out of a rock. The LS5 is the ticket here if you want to stay with the ARC product line. Otherwise I would seriously look into the BAT VK3i and VK5i models. Spending something like $700 would make a lot more sense in a preamp model change than a dozen or so new parts in your LS2.
John

I did some extensive rolling with my LS-2B when I had it. My favorites were
the German tubes, Siemens E88CCa and Telefunken E88CC. Be careful to buy
from a reputible source as these tubes are expensive and are also
counterfitted. Also, the modification I had done by Great Northern Sound was
well worth the investment (level 1 mod...about $700). Did a comparison
side-by-side with a friends stock LS-2B using the same tubes. Definite
improvements in clarity and soundstaging. No associations with GNS.

Have fun!

Marco