It wouldn’t hurt to check Craig’s List or other local sales for a used 8x10. Most people do not ship these amps when reselling, so you can score great deals sometimes.
Not to dumb it down but “more speakers, more better” as a general rule of thumb, IMO. An 8x10 is better than a 2x15, in my experience, even though they are about the same size box.
The suggestion to get a 4x10 now and add a second one later was good, as long as that would work with your amp.
In my opinion, mixing speaker sizes doesn’t benefit you like people might hope it does. If you’ve ever seen the band Zoroaster, they have a wall of mismatched cabinets, and the band sounds like mud. Conventional wisdom is to have all the speakers be the same size, ideally with multiple cabinets they would all be the same size, manufacturer, and same rating.
I’m not an expert with gear but this is my kind of music. The best bass sounds I’ve heard were Indian (two Ampeg 6x10), Weedeater (two 2x15, one Peavey and one Sunn I believe) and if I recall correctly, Unsane had two 8x10s. That’s probably more than anyone really needs, but more speakers do indeed equal “more better” when the bass needs to sound absolutely massive.
There was probably nothing wrong with the tone of your Sunn speaker cab, you were just asking a subcompact to do the work of an earth mover. In order to feel the bass, you need lots of speakers.