Hi,
Unfortunately yes, that does sound very much like the tube is on it's way out.
Sadly too, the value of these sets are so low now that it would be uneconomic to replace the tube
On the positive side, you can pick up another Avant for very little money - less in fact than it would cost you to fix your existing set.
Regards
Michael
BV5, BV Avant, MX4000, MX1500
BS 7000, BS 5500
Penta III, Penta II, BL 4000, RL 60.2, RL 35, CX-100 Alu
BL 7000, MCP6500, MCP5500, BL 5000, Beo4 (x2), BL 1000 (x2)