Had the same issue with the Beats on my first gen 
Formentor, was expecting the Sennheiser system to be better but I guess not.For the first gen 
Formentor  ARS-
Shop  has a few plug and play upgrades, amps, speakers, subs and they present their work on YouTube as well. I got their sub which works great in place of the Beats one. Also I got the Match 5.4 dsp amp, powerfull great little thing,  which is plug and play with a specific adaptor but delivered the power mainly on the front speakers and the sub. Had to remove it as when parking the warning beeps out the front speakers were extremely loud. I m sure ARS offers  amps that deliver power evenly to all speakers, you have to contact them and they will 
guide you. They specialize in upgrading the VAG family sound systems.