The 2006 GS Spyder recently had some hifi installed. An Alpine CDE-HD149BT
receiver with Sirius. Audison speakers throughout except for the sub: https://www.audison.eu/
They fit, and are very high efficiency making the most of the receiver's 18wpc. They have very little bottom end for various reasons including design, mounting area, and electrical setup. They seem to want 80 to 100hz crossovers which is not uncommon for 6.5"or 5.25" drivers in auto applications.
The initial installation ran through installing a dedicated subwoofer amplifier, a JL Audio XD300 XD300/1v2 - Car Audio - Amplifiers - XD - JL Audio
, mounted in the area formerly occupied by the CD changer. It was wired to the OEM subwoofer. The OEM subwoofer is a dual-voice-coil model, which explains the four wires leading to it. The reason it is dual voice coil is that it then uses two channels of the Auditorium OEM amplifier, for the equivalent of about 35 watts into that speaker, versus half that for all of the other speakers. Bass drivers need more power. The JL give the right amount of power for this application --- the Auditorium does not.
By installing the dedicated subwoofer amp, the Auditorium amp is relieved of two of its six or eight outputs (I am not sure if the tweeters are run on separate channels, they probably are not). In theory that should allow it to better serve the channels that remain in use.
Hoping that the factory subwoofer driver (speaker) would work well with all the other new parts, I left that alone. The two voice coils were paralleled and driven in parallel -- the amp is good down to 1 ohm loads.
Overall: Sound quality was fair. Clarity was fair. Imaging was good to very good, front-back and side-to-side. Bass was indistinct and not tuneful (pitches were not rendered). The "thump" quality of the bass (think kick drum) was slow to the beat and with overhang, indistinct. One might say tubby. It was not loud, and it certainly was not deep. I could hear nothing that sounded to me like sub-50hz information. I could not make it better adjusting the bottom two or three bands of the 9 band, built in parametric equalizer, nor with the crossover system including various roll-off points and rates.
The Focal subwoofer ALMOST fits the oem opening -- the opening needs to be shaved about 1/8" all around. The oem screw receivers ALMOST fit but don't force it -- new holes are needed and new screws. The Focal cover ring and grill are essential as the speaker has more cone travel than the oem driver and the oem grill will interfere with (hit) the driver as it works.
The sound with the Focal installed was revelatory. The entire sound picture -- all frequencies except perhaps for the top two octaves -- became focused, timely, articulate, clear. (The top two octaves were very good before and after. Better integrated into the entire sound picture, after.) The bass was very timely, excellent pace -- see the missives published by Linn and Naim lovers for more on that point. The bass is reasonably deep, perhaps to 40 or 45 hz, covering all of the frequencies of the electric or acoustic bass. I am hoping that with the reserve power available, and the capacity of the speaker for low distortion output and significant excursion, I may be able to equalize a bit better to go a little lower than 40hz.
It is not a thumper. It will disappoint drum and bass enthusiasts, probably reggae enthusiasts at least prior to the first splif. For jazz, most classical (forget the Organ Symphony), C&W, bluegrass, it is on the money and as good, clear and loud as one can wish for.
It write this to offer the opinion that tuneful, articulate, relatively deep bass capability makes all of the music sound better, as it has here for me. The Focal speaker can be bought for about $350 delivered. The amp is about the same price. The factory enclosure is up to the job (it does not rattle or otherwise ruin the bass output) though it has size limitations that will preclude 30hz bass and lower, maybe 40hz bass.
I am not aware of any other driver in production that would present an alternative to the Focal driver. There is a 5" subwoofer on ebay for a lot less that is also a lot less efficient. Someone may wish to try that and report.