BTW want to clarify 2 things "in case":
#1 -- I would certainly "go foam" over HD2 if price were identical, but so far as I can see I'm better-off having a pile of a dozen fresh HD2's to swap liberally, than cleaning my $60 HiFlo setup... the price difference for the sub-2% gain of foam should be percentages, not magnitudes, but am actually still seeking "raw foam" 60-ish porosity because if you had a block of that you could simply make infinite filters, I mean for me "going foam" means several 660's, a couple 2511's, and then looking at my other saws cussing them out for not being popular enough to have premade foams available ;D OK guess the 590 does, so I'd be a couple hundo $ in for pre-cut's...
#2 -- When I speak of "cut air boxes", the airbox casing has that 'lip' that mates to the carb 'wall', and Stihl has 3 ports of entry (and ask you to block 1 of them), anyways when I speak of "opened & filtered" I mean large DIY Frog-Skinz type openings like this:
(I use cheeseclothe-type paper filter media on the inside-facings, typically w/ 5-min-epoxy or a glue-gun, the 5min epoxy works better IMO and while yes it's easy to swap-out these papers it's surprisingly easy to just clean them, can't recall the last time I had to swap one even on my daily use climbsaws)
BTW
@Ronie if you didn't already opt for one of those (sexy,overpriced, small inner-area) XL clutch-covers, you can actually mod the OEM quite effectively:
OEM cover has 25mm of lateral space from powerhead, and a >4mm flared bottom that flares inwards & outwards equally. You can use a flap-disk and basically flatten the inner a bit, you never make anything under 1mm thick or anything but you gain over 2mm which is 10% lateral increase, also the OEM chip-flaps are large & chunky - and bigger on the XL covers - you can make them easily or, as I ended up doing (not pictured) simply take the OEM, mount the top peg-hole onto the
bottom peg, cut-off whatever excess protrusion you do
not want, epoxy to bottom peg & grind-off top peg, now you've got a much larger internal area as well as 'exit area'