i think the problem is just tuning, with the helps from experienced installers im confident the current system can sounds great if tuned towards SQ
why change things when a little finetuning can solve the problem?
just visit any SQ-knowledgeable shops in your area & ask them to tune the system

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any amps can sound clean if set below audible distortion level
human cannot hear distortion anything below 1% for sinewaves & even 3% for musics
cheap amps, expensive amps all be be SQ amps as long as it play below 1% THD
only thing is cheap amps usually overrated in terms of power ratings

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sub SQ is usually related to box design
a chapalang sub can sounds great if mated with a properly designed box based on the sub TS parameters
is your box custom-made for your sub? or a universal box?

whats your crossover settings? your highpass? lowpass?