I always try to deflect it towards the band - "Glad you enjoyed the band" or "thanks for staying and watching the band" - the bass is just a part of the whole, especially in our setup. I don't do anything clever or technical, just try and play it the way Phil did.
That said, a huge Lizzy fan who reeled off the times and tours he'd seen them on paid the whole band a massive compliment when he said any one of us could have could have played in Thin Lizzy - I didn't knowhow to react to that except for "Thanks, that means a lot"