I have a SK68 that can run 3AA using an extension tube. It can also run on one 14500 battery (haven’t tried) or 1AA (2aa works fine too).
Question: Since this light can run down a 1.5v AA battery does this mean that if I use 3AA, it’ll run each down below 0.5v? If so, it seems great for alkalines, but I read that this is bad for rechargeables.
Do not know the figures,but think that most battery types has
a level of voltage where there is very little power left.
Tell us if it runs at all on 3 0,5volt alkalines?
It is said to be a budget forum,thou I sometimes doubt it. 
Most of the sipik clones i own have no low voltage cut-off. Pretty much they work like a joule thief and drain the battery till there’s nothing left (and the output is a super low moonlight) on any of my alkalines or nimh. this is bad for nimhs, so moral of the story is don’t run the light so long that it dies on you with nimhs too much, regardless if in single or multi cell arrangement. I think one of my 3-mode clones will start flashing when the voltage gets to like 0.8 or something, but i’m sure at least 3 or 4 of my one modes just go till they’re out of all juice.
I have tried extension tubes from a few lights but haven't yet found one that fits my SK68 or clone.
Could you tell us what tubes you are using and where you got them?
As far as running AAs down, dropping to a low voltage won't damage NiMH per se, but you run the risk of cell reversal if your AAs are not evenly matched. Reversal is very bad for them!
If you have a decent charger such as the MAHA C9000, you can actually analyse the batteries for capacity in order to match them. If you don't, there's often no way of knowing if one cell is in reversal. All you can do is be very attentive to changing the cells when the light starts to dim slightly if you are running in two/three cell configuration.
@ gadabout: See the post after yours from Lennart.
I’ll toss some cheap batteries in mine this weekend and run them down. Will post results.
On all of my Sipik clones its either batteries going empty and starting to flicker, but its when they are really drained OR the light will not switch on the first time I press the switch, even tho the light might output reasonable amount of light, its a sure sign, that its soon going to be dark.
Tested with battery charger and it turned out to be true, battery was almost empty. Not sure, perhaps it was with Vikicompany’s Ebay 3 mode clones with silver zoom ring or maybe with some other clone model, dont remember, sorry, I have clones from 3 or 4 different sources and Im using all of them once in a while, so this data might get mixed up 
But as others have pointed out - unless you run your NiMh’s COMPLETELY dry, there shouldnt be any risk of damaging your batteries.
Perhaps some of our battery pro’s have something to object, are all batteries drained equally in such setup?
oooohhh... a thread to my likin'
I've had two duracell alkalines in a 3-mode by my bedside for months, yes months. Everytime I check the voltage, it's at 0.8v each, but still quite useful as 2AA nightlight... those batteries simply refuse to die, like a self-regenerating starfish. so I had this bright idea that since the tubes work for 3AA, why not see how low those alkies will go... so what if i get polarity reversal, big deal. So I add another extension and another 0.8v duracell into the mix, and go take a shower with this 3AA 2.4v sipik. worked well. I turn the light off and wonder if it'll go another night.
the following day I try the light... nothing, maybe one battery is dead. unscrew tailcap, can't pop batteries out. panic. unscrew the sipik, OK, one battery falls out of the sipik, but the 2 in the 2 extensions are frozen, and the last joint is welded shut. I finally manage to unscrew both extensions using two plyers. surprise surprise, battery leaked and ate half the paint inside both extensions. half the threads are gone too. so now I have these cleaned two extensions with rough bare alum inside, afraid it might scratch the battery wrapper if I don't polish it, so it's sitting on my desk. no problem, i still have other extensions, and they'll see only LSD's from now on. :)
ohhhhhhh…don’t do that.
Yep, running your alkalines down as low as possible and leaving them unattended will prompt them to piss in your battery tube every time.
Well, seeing how it went for you, I stopped testing when my batteries went down to 0.4v each…. and tossed them out! Thanks for taking a hit for the team… I would’ve probably left mine to run unattended through the night and ended up in the same boat! 
The light was still going good though. It was low obviously, but still very usable.
Edit: Just noticed your screen name. Nice.