Rob /Rossy oem Gransport rear boxes have valves as standard, its real easy to fit either Gransport boxes or any valved system to a 4200 and either have it operated integral with the sportmode or independantly , the solenoid valves are available through any dealer or eurospares , i believe Larini now do there own kit
Never ever noticed any cabin resonance/boominess with the Larini's ( but i have got the full entire system)
On my 4200 i had Tubi 's lovely burble and not toooooo intrusive generally , but at 2200 -2500 revs it caused slight resonance which used to make the handbrake handle vibrate ( kaz was very gratefull at times lol ) 2500 was cruisng speed around here and occasionally got on my pip , remedied by some rather unessassary gear changes hahahahaha
My advice pay the extra and go for a valved system, least when you want to be a model citizen and civilized you can
For those that mght be interested i have a rather shiny set of Gransport rear boxes sitting on a shelf in my garage
regards loz
regards loz