Connection
Thunderbolt interfaces come in two generations. The Apollo Twin and Twin MkII have Thunderbolt 2 ports, while the Twin X and Apollo x-series use Thunderbolt 3. A Thunderbolt 2 unit on a newer Mac needs Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter and a real Thunderbolt cable, never a plain USB-C one. Windows is hit and miss. UA wants a Thunderbolt 3 or later port, says Thunderbolt 2 Apollos won't run on Thunderbolt 4 PCs even with an adapter, and doesn't test AMD systems. MOTU's 828es, 8pre-es and 1248 also have USB, so they work without Thunderbolt.
9 models we cover.

Universal Audio
Thunderbolt$440–$625 typical ask
38 on eBay
Universal Audio
Thunderbolt$800–$975 typical ask
9 on eBay
Universal Audio
Thunderbolt$750–$1,025 typical ask
83 on eBay
Universal Audio
Thunderbolt$1,450–$2,000 typical ask
31 on eBay
Universal Audio
Thunderbolt$1,950–$2,525 typical ask
22 on eBay
Universal Audio
Thunderbolt$2,575–$3,000 typical ask
17 on eBay
MOTU
Thunderbolt$500–$525 typical ask
1 on eBay
MOTU
ThunderboltSpecs and what to check →
MOTU
Thunderbolt$825–$1,125 typical ask
3 on eBay