This quote from an old Hi Fi World site link for the E70, it mentions the E30 in despatches at the end, but these values may be out of date by now:-
"Nowadays mint pairs of E70s go for £150 (but pay half that for old nails), while their smaller but equally worthy E50 brothers cost £100. In 1979 the monster E90 appeared, the reinforced PRO version of which ended up as a PA 'speaker in many schools and cinemas.
The subsequent E30 and E20 were also reasonable enough but lacked the E70's star quality - pay £200, £70 and £50 respectively for mint examples. But whatever you do, don't overlook Wharfedale's Efficiency series - to paraphrase Jarvis Cocker, your system will whizz when sorted for Es!"