This may be a bit long and wordy. I'll try to keep it as short as I can and do justice to the issue.
1. There are a ton of guys over at TitanTalk who have made mods to their sound systems, from minor to major to complete replacement and everything in between. Do a search and check the threads there. You are likely to find just about any info you might want.
2. I'm no fan of Bose, but I do have the audiophile bug and use professional grade equipment (although I'm an amateur) to set up my high end home "stereo" and my home theatre equipment. I have posted several places (I believe here, TitanTalk and Nissan Off-Road) about my testing of the Bose system in our Armadas. My findings are that despite my anti-Bose bias, the system is remarkably flat throughout the midrange and high frequencies.
The only problem I found in frequency response was a very, very bad, gross, horrible (I can't find words strong enough in a family forum) over-emphasis of the upper and mid bass, consistent with cabin resonance and perhaps poor choice of cross-over frequency between the sub and the door speakers in the mid bass region, around 80-120 hz range.
In my truck, if you turn the bass all the way down to "-4" or "-5" it flattens out pretty nicely and still has strong bass coming from the sub. After making that adjustment, just to make sure my sound level meter and test tone CD were not lying to me, I played a CD of cathedral pipe organ music and the sub would still rattle the windows.
I've replaced the factory junk in almost every vehicle I owned, but I'm leaving this one alone - that one adjustment got me as close as I've ever been able to get with high end aftermarket stuff in a car/truck environment. Now, if you need a lot more power, that's another issue.
3. There is still confusion regarding who actually makes our Bose head unit. I posted the TSB up above and noted that Nissan seems to be dealing with Panasonic to obtain the units for replacement. But that does not mean that standard Panasonic or Clarion plugs are used. I don't know.
4. Let me confuse the manufacturer issue even more! If you click on my link above and read the TSB you will see a photo of a typical "sticker" on the unit. Note that it has a "Visteon" part number. If you "Google" that name you are likely to be in for a shock. Here is their web site.
http://www.visteon.com/
Visteon is, get this, a spin-off of Ford Motor Company!!! It is their former auto electronics division, like Delco for GM. It remains the major supplier of auto electronics for Ford, but also supplies stuff for a lot of other car makers. Gasp! Ford stuff in our Nissans! Maybe Visteon works in coordination with Bose? Maybe it outsources the head unit to Panasonic, hence the separate Panasonic part number for that? I just don't know what in the world to think of a Visteon label inside a Nissan vehicle.
But because of this, if you can't find the answer from all the Titan guys over at TitanTalk, then the advice given by others here to take it to a professional shop for the conversion kit to operate the Sirius radio is "sound advice." But if they try to suggest you change out the whole system because it is junk, you might want to spend $40 on a RadioShack sound level meter and about $30 on a test CD and do your own testing first. Once you get rid of the cabin resonance, you might be surprised at how good the system actually is.