Truthfully, the best budget 15k SQ I've heard will be on a secondhand 9887 + 5,000php speakers on active configuration with a 50hz cutoff on the midbass
SQ by itself compromises of three criterias. Sadly, most of us simply think of it as having good tonality. But it compromises of three things: imaging, staging, and tonality. Unless you plan on throwing a DSP, you can kiss imaging and staging goodbye. But in doing so, you'll be spending a lot more even if you go for a "budget" build.
So my recommendation will be quite simple on a 40,000php budget:
- Head unit (20k)
- Speakers (5k)
- Amplifier (10k)
- Subwoofer (5k)
Then extend a bit more for wirings,deadening, and other miscellaneous stuff. If you can get "packages", all the more better so you know what to budget for.
Your aim is to cover the entire sound frequency spectrum. From 40hz to 16khz. Your separates will handle the 80hz upwards while your subwoofer fills in the lows. Through experience, I would say your most immediate jump will be the speaker change.. and the inclusion of the subwoofer -- because nobody really hates bass.
If you're in need of trunk space, just make a small box. Even a 0.7-1cu.ft sealed box will sound better than an underseat subwoofer.