Is it possible to keep the stock head unit in my car but replace the stock amp and/or speakers with aftermarket equipment?
YES.
Upgrading sound system while maintaining stock PREMIUM HU:
The stock premium head unit has no internal amplifier. Instead, an amplifier is located in the trunk, under the left black support brace.
Other than amplifying the audio signals coming from the stock HU, the amplifier in the premium system also adds heavy equalization to the output signal going to the speakers.
Because the external amplifier adds the amplification and equalization to the audio signal, the audio signals leaving the stock HU can be treated as a very low voltage preamp signal. By very low voltage I mean that the audio signals as they leave the stock HU and enter the stock amp in the trunk are < 2V.
In addition to the low voltage of the signals leaving the stock HU, the signals are also not your standard type of signal. Unlike conventional single-ended (that's what they are called) signals, the G35 sound system utilizes differential-balanced (that's what they are called) signals. Without going into deep explaination, differential-balanced signals do not connect one side of the audio signal to chassis ground. Because of this, balanced-differential signals often times help to eliminate any voltage sensitivity between one input terminal and ground and greatly reduces the incidence of alternator whine in a typical car audio installation.
Most aftermarket amplifiers expect RCA-style connectors/connections on the input jacks. When bypassing the stock amplifier to connect an aftermarket amplifier, you will quickly realize that you are working with speaker wires, not RCAs. There are two ways to convert the speaker wires located in the trunk (where you are bypassing the stock amp) to RCAs (to connect to your aftermarket amp's input jacks).
1. Line Output Converter (LOC). This device simply has speaker wires coming out one side and RCA jacks on the other. After splicing into the appropriate input wires coming from the stock HU, you will be able to connect the RCA jacks on the LOC to the RCA input jacks on your amp using a standard RCA interconnect cable. NOTE: If your amp does NOT accept differential-balanced input signals on its input jacks, you MUST get a LOC that is capable of converting the differential-balanced signals in the G35 to the standard single-ended signals the aftermarket amp is expecting on its input jacks. Soundgate makes an amp that does this.
2. Splicing braided/twisted RCA cables (PREFERRED IF YOUR AMP ACCEPTS DIFFERENTIAL-BALANCED INPUTS!!!). If your amp accepts differential-balanced inputs on its RCA input jacks, all you must accomplish to connect your amp is to convert the speaker wires in the G35 to RCA-style connectors that your amp requires. While a LOC will accomplish this task, it is overkill and adds unnecessary complexity and possible noise into your system. If you have ever cut open a standard RCA cable, you will notice that there are two conductors, one of which is often VERY tiny. Splicing such a tiny conductor to a speaker wire is very difficult and does not yield quality sound for your inputs (which the entire system is dependant upon!) Look for twisted RCA cables, or braided RCA cables. These use thicker gauge wiring for both conductors. INOXS makes cables that work well for this.
Upgrading sound system while maintaining stock STANDARD (Non-Premium) HU:
The stock standard head unit incorporates an internal amplifier which powers the car's speakers. To upgrade the standard sound system while keeping the stock standard HU, there are two popular options:
1. Choose an amplifier which accepts both speaker level inputs and differential-balanced signals (on the inputs).
2. Use a speaker-level-to-preamp converter such as
this Scosche. Use LOCs or twisted/braided RCAs (if your amp supports low voltage inputs) to convert speaker wire in trunk to RCA for amp inputs. Your amp would still need to support differential-balanced input signals, unless the LOC converts them to standard single-ended input signals.