To accomplish the 'larger than life' main pair picture of the marimba (before adding the string spots) ....would you recommend a relatively low height for the main pair....say 5-7 foot, and not too far in front of the instrument ? Of course you'd want to be far enough away to minimise the motor noise...
My inclination is, if it's a classical ensemble, to bring the main pair down to eye level or even lower and then physically move the marimba forward until it is front and center in the main pair. You likely won't even need string spots if you can arrange things right.
If there is motor noise then it's a vibraphone and not a marimba and that is a different problem.
--scott