Understanding how your speakers and subwoofer behave inside your room is key to getting the most out of your system.
Only with measurements, you can confidently make adjustments and set EQs, crossovers, phase, etc.
Luckily measuring equipment got very good and inexpensive over the last few years.
miniDSP UMIK-1 - the most commonly used USB microphone
miniDSP UMIK-2 - updated higher-end version of the UMIK-1
Dayton Audio UMM-6 USB - alternative to the popular UMIK-1
Earthworks M30 - something of an industry standard and one of the best XLR mics. Not cheap but worth it if you are really serious about measurements. Also needs a high-quality microphone pre-amp