Thanks for mentioning the space, midbass and the number of boxes
The UM18 is very popular as far as 18 inch drivers go. Best suited for deep HT bass in a large ported enclosure, not a mid bass monster. Next would be the PA460, usually placed in a VBSS that offers a cheap solution that has mid bass and can dig, but excursion limits potential output. Something to consider in a 5000+ cuft space.
For 21"s, the B&C 21DS115 is a popular 21" driver. It will take 20mm of excursion and has a high sensitivity of 99db. That gives it versatility being able to reproduce low bass and hit hard in the mid bass region. The price tag makes it less of a budget option, but if you want to throw two in a couple ported boxes and drive content in a large space at reference, it is a very compelling option. B&C has more variants of 21" drivers, that have different strengths and weakness, the DS is nice balance for HT use. LaVoce SAN214.50 21 is another option that is a bit cheaper and from the WinISD sims it handles very much like a 21DS115. However, it only comes in a 8 Ohm flavor.
I am very pleased with the 21DS115. My friends that come over note how well it blends and sounds effortless. It is in a 11 cuft 18Hz tuned box, it can nearly hit reference on everything I have tried so far, just a few exceptions. The room in which it is in is roughly 6000cuft I want to say 11 ft from the MLP.
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, they sell flat packs for a lot of things. They would be considered a newer development.