Bass to mids fill

I was wondering that with my system i am tuned to below 35 hz cause i have 3 low pass filters set to 35 hz. When i play music i can sense a gap between the bass and the midrange. I want to fill in this gap. I recently purchased a pair of 6 X 9s and 5.5 inch seald basket speakers for my mids. I am thinking of putting these in a very small (just enough space for the speakers) well seal box so that it would be able to handle the lower bass frequencies with little distortion. Will this set up work or are there any other ideas to help me with the fill effect?

IMO 6X9 speakers are some of the worse sounding you can possibly use.  If you like them, then great, but a good quality 6.5" coax or component set will blow them away - in general.  If you are using your 6X9s as your mains and crossing them at 150Hz (a good idea, by the way) you have a huge "hole" between 35Hz and 150Hz, which means you have no low bass in your system.  Not a good setup at all.  You really need to consider dedicated mid-bass drivers.  I suggest a good set of 6.5 or 7" woofers in kick panels in the front of your vehicle, driven by their own stereo amp crossed between your sub and mains.. 

As far as your sub-bass, you are robbing yourself of most of what your woofers can do by crossing them so low.  There is VERY little information below 40Hz.  If crossing them properly at 80 to 100Hz results in too much rattling, etc. then you need to improve your installation.

prodigal, the reason why your 6x9s in free air distorted is most likely due to poor tuning. are you using a factory deck or aftermarket? If its factory, adjust the fader so that it is front side bias. the rear speakers are only for rear fill. if its aftermarket, dont adjust the bass so its maxing out. in fact, you really should keep it at 0 or in the middle.

A sealed enclosure is definitely NOT what you want for the 6x9s. Like it has been mentioned already, you will lose all bottom end from the speakers. this will create a gap in the frequencies. You need to turn your subs up to atleast 80hz and invest in some sound dampener. that will take care of all of your rattles. Everything you have told us so far tells me that you have a poor install and dont want to do the proper steps to make it sound great. Great sound takes time and money. it is an investment that will yield you great results in the end if you take the time to do it right. Im not trying to put you down, just tell you the truth.