I have a 2012 my SDV6 with the standard HK system which is perfectly suitable for general use now I fixed the farting sub in the tailgate and stopped the rattles. However, it is simply not loud enough sometimes so the volume ends up round at the high 40's.

Is it possible to fit an additional power amp (many available with speaker level inputs) so it takes the speaker line outputs from the rrs amp module and gives them some bite ?

Have a look at Audison

https://www.audison.eu/products/ap8-9-bit/#group-1

There are several different models so you will need to research what model you would need for the amount of speakers in your system.

Not the cheapest but get rave reviews and are designed specially to intergrate with OEM systems  2003 BMW M5 - 5 Litres of V8 Optimax Slurping Goodness in Carbon Black

2013 SDV6 HSE Black Edition- Orkney Grey with Ebony & Ivory. LR Facelift RAI, Genuine G4 Expedition Rack, 4x Warn SBD-160 Spotlights, Warn 9.5XPS Winch, Mantec Underbody Protection and Sump Guard, Full Easy Lift Suspension Module, LR Light Guards, Light Force HTX230 Hybrid spotlights, 18” Compomotive PD1881 on 265/65/18 BF Goodrich KO2, ARB fridge, TuffTrek awning 😎

Not what you were looking for  2003 BMW M5 - 5 Litres of V8 Optimax Slurping Goodness in Carbon Black

2013 SDV6 HSE Black Edition- Orkney Grey with Ebony & Ivory. LR Facelift RAI, Genuine G4 Expedition Rack, 4x Warn SBD-160 Spotlights, Warn 9.5XPS Winch, Mantec Underbody Protection and Sump Guard, Full Easy Lift Suspension Module, LR Light Guards, Light Force HTX230 Hybrid spotlights, 18” Compomotive PD1881 on 265/65/18 BF Goodrich KO2, ARB fridge, TuffTrek awning 😎

It could be. Certainly the concept is right, as ever it turns on the price. Funnily enough I emailed a local "specialist" on this (in the west mids where I am working at the mo) and the cheeky s tried to tell me it would cost £5k cos "these are complex vehichles and systems." I had more in mind 300 quid fitted ideally !

They are expensive but they have been designed with oem installations in mind. I think that so called specialist is talking bull

After speaking to a number of companies there are great gains to be had by just sound proofing the doors on the insides to make them less tinny.  2003 BMW M5 - 5 Litres of V8 Optimax Slurping Goodness in Carbon Black

2013 SDV6 HSE Black Edition- Orkney Grey with Ebony & Ivory. LR Facelift RAI, Genuine G4 Expedition Rack, 4x Warn SBD-160 Spotlights, Warn 9.5XPS Winch, Mantec Underbody Protection and Sump Guard, Full Easy Lift Suspension Module, LR Light Guards, Light Force HTX230 Hybrid spotlights, 18” Compomotive PD1881 on 265/65/18 BF Goodrich KO2, ARB fridge, TuffTrek awning 😎

Yes, good points. Sorted my subwoofer by doing exactly that and have no prob changing out drivers for better quality in due course, but for now it really is a volume thing. The drivers are good for a bit yet but the amp is running out of puff so either a gain stage pre power amp or a slaved power amp off a speaker level input seems the best.