best might be two 8.4s in series as used by the Nautilus Extreme in the US . But the 230v 8.4s are hard to get a expensive so far .
Two 6.6 ... you need to make spacers , its easy enough . I have one fitted but got too lazy to fit the other .
Electro 5.7s , you wont run the top 8.5 amp versions on one cord .... I have sourced the tapered 7amp version from the states and use one with the 6.6 as of now . Can i tell the difference with a 6.6 ... no .
Your easiest upgrade is like Robin said ( not the switch to parallel
) but to fit two of the HD electro motors .... its comes with one so you just need another ... you gain one one HG , i measured it .
Its worth it in that its a simple upgrade , but no two different motors are going to do anything dramatic .