To be 100% honest, I don't know that many serious harp players the really like the tweed Deluxe for harp.
The 5C3 is a bit different of course.... I don't know if the current run of Weber transformers for the 6V6 amp can hold up to the 5881s and a 5U4... I would think so but ???
I'd ask them first.
You must know they do sell a 5C3 P 6L6 amp kit that you could convert to harp playing.
For harp playing, I would bias the power tubes colder then the 250-270 ohm resistor normally found would allow.
Speaker for harp... it depends on your playing level, technique and the mics you will try to use, but you can't go too wrong with a 12A100-O or 12A125-O from Weber... other speakers from other vendors are just as good but sound different. Do another search on Google.