The overwhelming thing is that there is ALWAYS the next level, and that scales are logarithmic. Some chase the decimal point in the THD. Others chase decibels - I tend to fall in that category. Build a PA rig (car system, etc.) that can do X dB. At 30 Hz. Then you want one that can do 10 dB more. Ok, maybe do-able. Then you want the next 10dB and you've got to start re-thinking everything. Then the project stalls - perhaps for years - but never goes away.... And at some point the next 10dB will require more time and money than exist in the entire world.