Is there a pre-out (low level) on the Bolero stereo? i wanted to add some extra bass to the system probably in the spare tire space.

Id like to know if this is also available on the Columbus too at all? Some extra bass is always welcome!

Not one on the Columbus. I fitted an active sub in the spare wheel, which has completely transformed the bass: which I previously felt was missing in the factory speakers. Have a look at the pioneer wx610a. The installation is invisible, and you get to keep the spare wheel (the jack and tools go into the supplied bag, and store these in the side compartment). There are some good prices through eBay and amazon from a couple of German ICE sellers, because Sterling is strong against Euro at the moment. Expect £100ish for fitting, as there's a lot of trim that needs to be removed in that big old car...

You could consider an amp which has high-level outputs. I fitted an Audison SR4 to my Columbus which improved things immeasurably. I use two channels to power the front components and the other two channels are bridged to power a sub. Admittedly, this is in an Octavia, but that shouldn't make a difference.