Hi Caine.... firstly welcome to the 'other' side - that is Losi ownership, secondly, you've made a couple of great purchases in aquiring your two DoubleXs... these truely are great buggies (my fav of all time) & I think you'll be pleasantly suprised with how well they go compared to both the B4 & 22 - they will certainly feel easier to drive than either of the modern cars.
Hop-ups wise there really isn't anything that will make a major difference performance-wise... the cars really were that good out of the box! The only two parts that might be worth trying in order to improve the reliability/strength of the car is to get hold of an ali brace that ties together the inner hingepins on the front along with a strengthening plate that sits on top of the gearbox & spreads the load of the two fixing screws that go through into the bulkhead - these are both Trinity 'Kinwald' parts (can't remember the 'TK' no off hand).... I have seen gearbox cases & front bulkheads fail & these parts can only help. Also the RPM gear cover is a much more durable item compared to the original Lexan part.
Both the original XX & CR were both very competitive on the track & indeed some people prefered the original car over the CR but the only real difference was the change from inboard to outboard toe on the rear.... I prefered the CR car simply because the two-part 'Tee' peice seemed a stronger assembly.
Many parts from your 22s predecessor... the 'XXX CR' can be used to keep the XX going... all the gearbox internals including the diff assembly fits into the XX casing, watch out for the 'BK' XXX parts though as I believe this car had a different internal ratio? Also although the basic design of the diff remains similar between the XX & XXX some parts have changed during the evolution of the XXX - I have an original XXX buggy which shares all of its diff parts with the XX but I also have a CR2 (mid motored CR XXX) which has different out-drives/thrust assembly/tension spring & securing nut. The shock cartridge design has also remained unchanged since they upgraded the notoriously leaky seals on the original JRX2 - right up to the XXX CR. The threaded bodies on the later XXX shocks only allow for more precise adjustment, personally the clamp collers are just as good - off road racing doesn't require such accuracy
Other XXX parts that can be used include bearings, Spur gears, wheels, axles & driveshafts, rear hubs & early XXX (narrow VLA) rear wishbones work too on a XXCR
Its the parts from the later XXX buggies that seem to be still stocked or supplied by model shops.... I tend to order the spares for my CR2 from DMS Racing.
With regard to the Hydradrive... generally back in the day we tended to favour the friction slipper as it was simpler & less messy - also with 1700 SCRC cells & limp brushed motors we wanted as little rotating mass as possible. However with brushless power & efficiency r-mass is less of an issue & I've seen quite a few guys running the Hydradrive on their Atomic Carbon CR2s - it is reported to really soften out the ride & make the car easier to drive especially on really bumpy/rutted tracks as the unit damps the 'shock' forces through the drive-train. Those guys I saw running this unit were running 6.5s etc so I don't think there is a problem with the durability?
