Basically it depends on what type the chainrings are - 104mm BCD 4 arm fitting or 94mm BCD 5 arm Compact drive.
Or if the Middleburn chainring is an "Uno" then it will only fit the RS-7 and RS-8 cranks anyway.
Most likely to be 104mm I suppose, in which case you have a huge choice of cranks you could use. Certainly the current LX Hollowtech cranks could be used as they are 104mm 4 arm, but you could also look at finding some older square taper cranks and use them with a Shimano UN72 bottom bracket.