Selling my UL Twin

[Sauerkraut]

I've finally decided to sell my 1979 135watt UL Twin Reverb. It's too loud and impractical for me so I want to replace it with a silverface Deluxe Reverb or a Vibrolux Reverb. But before I do so I need some advice from you guys.

Its specifications are:
- Original speakers (one '76 Rola and one of that other type of speaker they put in all the '79 Twins, oddly enough it came out of the factory this way).
- Circuitry in good condition and hasn't been tampered with.
- Original pre-amp tubes (General Electric).
- Crappy 6L6 powertubes.
- Tiltback-legs and casters.
- It's got some tears in the vinyl so it doesn't look too good, but not worse than most Twins from this era.
- and the original booklet, not that it's worth anything.

pictures can be found here: www.fotki.com/sauerkraut. (of course I can make it look a little better than in those pictures, with some effort)

This is how I bought it 3 years ago from the first owner and I didn't change anything, I just had a technician do some maintenance and that's it. I paid 750 euro's for it back then, which probably wasn't a very good deal but I didn't know shit about these things back then (and I'm not much wiser nowadays).

As I need the money (especially if I want a Vibrolux) I don't want to sell it for less than 750 euro's, but I'm afraid that I can't ask that much for a worn Twin with shitty powertubes.

now here's my question: Would it be worth it to buy four of these; http://www.watfordvalves.com/product_detail.asp?id=1392 before selling it? It would cost me about 90 euro's, do you think I could add those to the value of the amp? Would it be reasonable to ask 850-900 euro's for a physically worn '79 Twin with original speakers, original pre-amp tubes and good powertubes?

edit: keep in mind that prices in Europe are higher than in the US, so if you think I'm asking a ridiculous amount of money, I'm not.