If the temperature of the liquid gas in the canister gets too close to the boiling point of the mix, there isn't enough gas pressure to make the burner work properly, and performance drops off and then fails.
As you use a gas canister, the mix is cooled off by the evaporation of gas from the liquid, and also the higher boiling point part of the mix evaporates off first. By the time a standard 70/30 butane.propane mix canister is half used, there's very little if any propane left in the mix. That means you won't be able to light the stove below 1 or 2°C, and it won't run well below 7 or 8 because of the cooling. The way round it is to warm the canister beforehand, which can take a while and be uncomfortable, and to use a fairly close fitting windshield so that heat from the flame warms the canister and keeps it warm (take care not to let it get too hot).
A remote canister preheat stove used with the canister inverted uses gas at the same ratio as the mix in the canister, That means there's always enough propane present to provide adequate pressure to push liquid gas through to the burner and to get the stove lit before you turn the canister over. The main thing is to use the canister upside down even when it's fresh, to preserve the propane content for later. The Karrimor Alpine stove is the cheapest remote canister/preheat stove around, now at £17 with piezo. Some of the expensive versions have a canister stand so you don't have to wedge the canister upside down with a boot, and they may have finer valve control when using liquid.
If you want a canister top burner, the Jetboil canisters are the best I've found relatively easy to come by. They are an isobutane/propane mix, and isobutane boils at -12 rather the -0.5 of normal butane.
The XTherm certainly feels nice and toasty, in so far as I can tell without having severely provoked it (about zero, which would have been not unbearably chilly on the original Neoair). The crossways baffles don't do anything to keep you on the mat if you are inclined to roll off, but it's the same as the X-lite or original in that respect.
All the extra internal baffles have an effect on the roll size as well as the weight.
with between 300 - 500 rechargages before I start losing power that's maybe 15+ years of use. I may well be dead by then! (only 7 more years to get my state pension)
Lithium Ion cells deteriorate with time even if you don't use them much, so I'd guess that you will have to replace the cell after maybe 4 or 5 years.
In general they don't like being run completely flat (though many are "protected" and have a built-in cut-off to prevent it), don't mind being charged from part-used, and if you store them unused over summer, that's best done at half-charged, somewhere cool.
IMO, a detachable accessory diffuser like on the HP15 doesn't count when considering the availability of a good flood beam. It's just too likely to fall off, get lost, get left behind or in the bottom of your pack, or break to be considered as long term useful.
I've no disagreement with a flip up diffuser being a very handy feature on a head torch, but it really ought to be a permanent and non-vulnerable fitting, like on the Myo RXP.
The low self-discharge AAs such as Eneloop (Panasonic branded now), Uniross Hybrio, GP Rcyko, Ansmann MaxE etc are definitely the ones to go for. Eneloop Pro (was Sanyo Eneloop XX) give a usefully higher capacity/run time, but I've never noticed any difference between the different brands for the standard 2100 mAh cells.
I'd suggest the following AA Chargers:
7dayshop 600LCD, £10 - independent channels for 1 to 4 cells, has a discharge function which I'd recommend using for the first 3 cycles of any new pack.
"fully" independent means mixed AAA/AA allowed. Cheaper chargers terminate charge independently, but won't allow mixed cell types.
18650 LI-ion cells give brighter torches with longer run times than AA torches of a similar size, but off-mains charging isn't an option, and the cells can go bang, which makes carrying a spare or two in your pocket less attractive. Pay full price (£8 to £10 each) as the cheap ebay ones are fairly commonly recycled from laptop battery packs, and only give a fraction of the claimed capacity (as low as 1/3).
I don't like CR123. The rechargeables only give half the runtime of the disposables so the advantage ove AA is less, and disposables are really only a sensible price off web bulk order.
I like the Fenix torches. For headband use, I'd suggest the LD22 (2xAA, 6h at 100 lumens) or the PD35 (2xCR123 or 1x18650, 10h at 180 lumen on 18650). For a headtorch, I'd get the HP25 (AAs are too big for battery on the front use, IMO)