I was going to swap out the stock speaker with the G8C, and the factory spec is:
Maximum approx. 20W RMS @5 ohms
So naturally my inclination would be to get the 4 ohm speaker. But, should I think about the 8 ohm?
No … it sounds like it really wants a 4-ohm speaker.
Yeah, I already made the swap with a 4 ohm. Sounds a little better, but not that hot still. I got it for the mp3 input capability and headphones, so there is that…
Yes, a modern 20-watt SS amp with a single 8" speaker will never be more than a bedroom amp … jamming along with mp3’s included … hopefully it performs that duty nicely 
What would be a good WGS 10" speaker to upgrade to? The little amp sounds great just needs more volume and presence.
The G10C brings the most to the table here … but are you going to try to squeeze it in that cab? Might not fit!