Again IMHO OB and DB items may stack but it depends on the items, spells or effects.
MDC
I agree completely and we followed the same convention in our games which leads me to believe it was not just a house rule, and I recall an example in RM2 that shows how a PP multiplier would work with a PP adder for a realm working with a PP adder for a particular list. The PPs were used for very specific reasons, but allowing all of the bonuses to be combined into a massive multiplier/adder bonus was not allowed.
The PPx would only multiply the total PP inherent to the caster's ability and he could only use one PPx item (usually the highest if he was lucky enough to have more than one). The PP+ only worked a specific spell or list (for the example given). The PC has x2 PP so he gets, say 14 PP per day instead of 7. But he has a +4 Spell adder for Ranger's Ways list. He can only use those 4PP for specifically casting spells from that list, those PP do not come from the caster's 14 PP and those 4PP are not multiplied by the x2.
I don't think that was a house rule we used, it was a base rule from the game. We explained it away as the x2 multiplier affected the spell caster. The +2 item was imbued with the ability to cast 4 points from that list, and it only 'knew' that list and was therefore not subjected to the x2 as it wasn't really part of the caster's innate ability/aura/being.
I know we did stack items in this manner:
PPx2 = 7PP becomes 14PP
PP+2 (just general PP adder, not attuned to a list or anything like that) = 14PP + 2PP = 16PP
We added the +2 PP, but it was not subjected to the x2. Not 14PP + (2*2) = 18PP
We also restricted Magic Rings to two; one ring per hand. We said the magical auras were in too close proximity and the auras cancelled each other out, so no loading the hands up with ten +PP spell adders! LOL
We definitely stacked OB and DB, but had guidelines too. You can't wear two helms and get the DB from both. Simply having the +10 Sword on your body doesn't stack with the +10 sword you are wielding. General things like that. +5 Ring of Defense, +10 Shield, + 5 Helm of DB = +20DB? Heck yeah! We did that all day long.