Reading all of this makes me kind of suspicous....when such a price is mentioned i assume it's known what's gonna be repaired, the big questions is 'what' ??
Because with all the possible trouble with these tubes: heather problems isn't one of them (but you could have the exeption) but as mentioned is quite easy to check.
Now if the tube is gonna be replaced: B&O price (including different deflection board etc, needed for a replacement tube) used to be around 700 euros.
Don't bother having this done: the whole tv isn't worth more than 200,- (ex beo4) !!, getting the tube changed for a 50Hz avant is a waste of money...
On the other hand, as mentioned here, i also have serious doubts if your tube is the problem...
I don't want to talk 'bad' but i'd go for a repair guy who knows about Avants, for some repair places these tv's are quite ''mysterious''...:)