You don't need 3.5 Volt at the battery, a boost driver can boost up the voltage. The Solarforce 0.9-4.2V XP-G drop-ins work fine with two AAs.
The XPG needs 1.4A at the LED, ~2A from 2AA batteries.
The SST50 runs at 4A at the LED, ~6A from 2AA batteries. A boost driver for that level of power needs its own heatsink or it will fry.
A higher capacity 18650 can produce both the voltage and current (just).
A member once tested a Solarforce low voltage drop-in (can't remember if it is MrGman, TiForce or BigChellis) and got the following results.
Solarforce XR-E R2_________1 AW 2600mAh________ 218.5 ______ 1 sec
Low Voltage_____________1.6A at tail___________ 217.7_______30 sec
.09~4.2V input_________________________________ 218.5_______1 min
_______________________________________________ 216.2_______2 min
_______________________________________________ 216.9_______3 min
_______________________________________________ 219.2_______4 min
Solarforce XR-E R2_____ 2 NiMH AA Tenergy ______ 97.7 ______ 1 sec
Low Voltage_____________________________________ 96.2 ______ 30 sec
.09~4.2V input__________________________________ 96.2 _______1 min
________________________________________________ 96.2 _______2 min
________________________________________________ 95.4 _______3 min
________________________________________________ 95.4 _______4 min
Notice the light level boost with the proper battery.
If you look at the OP's signature he already has
iTP A6 POLESTAR, (Very bright P7)
Xeno EO3, (Tiny 1AA. Can be very bright with a 14500)
LumaPower IncenDio V3/V3+ GT Upgrade kit, (337 lumens in a turbo head)
Dereelight DBS V3 3SM R2 WD (turbo head thrower)
2AA is unlikely to produce the light levels of these especially the lux/throw numbers of the last 2. My guess is the OP was expecting more light than any one of them.
My suggestion of popping the SST50 drop in into the Dereelight head should provide more lumens due to the more suitable battery. And the Dereelight DBS V3 turbo head would increase lux/throw over the standard head of the Solarforce.