Really bassy JCM800

[Outlaws]

Back when the earth was cooling and JCM800's were new, the cabinets were loaded with G12M70 speakers. They had more top and a tighter low end.

I still think it's the tubes.

Ya the tubes would be something to check if you are not familiar with the recent history of the amp.

 

[NRS]

Also, I have never heard of the G12H being originally designed as a bass guitar speaker. They originally had two versions.

Correct, they had two versions. One version was a low resonance model for bassists and the other a high resonance one for guitarists. The modern Celestion G12H is recreation of the low resonance model.

It may seem like a rather weak bass speaker choice by todays standards, but remember, bass rigs were no where near as powerful, complex and huge sounding back then as they are today.