I understand from experienced JTM45 owners that they really love the contemporary Celestion speakers with this amp. Sometimes the 8 Ohm version Celestion T1220, with Pulsonic cone, are for sales at reasonable price. Might be suitable for 2x12, although powerhandling can be on the limit.
15 Ohm T1221 (for 4x12 use) seems to be more expensive.