You should use exactly whatever ohm load that the amp is rated for ... for 6V6 pair (power tubes) amp, I would expect that to rating be 8 ohm. Don't worry too much about the wattage- 15 watts won't endanger the speaker, but it might not sound like it would if it was receiving more output power (but probably better than the stock speaker).
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia