My first thought would be to leave the Celestions in there. If they don't sound to your liking, then decide which speakers would be better suited. But Celestions are fine.
Then keep in mind I have weird theories on everything. I think of a speaker this way; the treble frequencies come more from the center or right around the voice coil, and the bass frequencies come from the outer edge. This is why 'woofer' types have a rubber surround; for maximum pliability and excursion. So if the amp seems to lack bass response, I either look at breaking in the speaker more, or find a low wattage speaker with a thinner cone paper. That should allow the cone to move easier, and have more bass. Putting in say a 100-watt speaker would mean not much power pushing the cone, so not as much bass response. Each speaker would get 30-watts, so the lower the rating of the speaker, the better for my crazy theory to work. A 50-watt speaker is the most I'd try.
Now I look at the efficiency, or how loud they are. Some speakers are simply 'better' at converting input signal into audio. Others waste a lot of that input as heat. If you think the amp is plenty loud already, then a less efficient speaker could be the ticket. This means you have to crank up the amp to get volume, and the great break up with a low wattage speaker just equals great tone. You could try say the Jensen C12Q; 35-watts, and an efficiency of 94.6dB, which isn't very efficient. But they could sound awesome in your amp cranked up. Compare that to the C12N, which is 50-watts, and with an efficiency rating of 98.4db, which means a 4dB increase in volume. If you think the amp could be louder and have more headroom, then the C12N could be the ticket.
That's a start. Again, my ideas aren't necessarily sane and rational, but they make sense and work for me.