geloso g213 tube swap

Hey there,

I just got a geloso G213 http://www.radiomuseum.org/r/geloso_g213_a_g213a.html

The amp is very loud, but its sounds a bit "thin".

There are 2 12SL7GT tubes in it.
Can I swap it with a 6sl7, or maby a 12sn7?
I got 2 6sn7gtb tubes there, can I try those 2?

I read allot about 12a*7 tubes in harp amps, can i swap them? maby with a "adapter" or something.
And what brand would be the best/would you recommend

What would be the best preamp tubes to get more bass?

And it also got 2 6V6G tubes in my geloso.
I got 2 very nice tungsol 5881, Can i swap it for the 6v6g tubes?

All tips are welcome,

Greats from holland