How much power do I need? Find out using this method...

Thanx cal3713. Great link! Every audiophile should have a meter if just to test fuses and continuity in other devices and as you found out relatively 
decent ones can be had for little cash. Another nice tool to have is a dB meter to check volume levels. This one cost a whopping $15.00
https://www.amazon.com/Decibel-30-130dBA-Backlight-Flashlight-Included/dp/B07G8F8TK2/ref=as_li_ss_tl?SubscriptionId=AKIAJO7E5OLQ67NVPFZA&ascsubtag=3711
If you want to test the effectiveness of your tube traps just put on a 30 Hz test tone and walk around your room with the meter. If your variation is less than 3 dB away from the walls you are doing a great job.

Not really. If you calculate the power consumed at that particular volume knob setting you can double that power value and there will be only a 3 dB increase in volume. Even though it’s twice as loud it is almost imperceptible to our ears -- at least 6 dB increase in SPL will make it noticeably louder, which would be quadrupling that calculated power.

A slightly more accurate way of determining how much power is enough is to use the speaker’s sensitivity vs listening distance with an SPL meter. For example, if you sit 4 meters away and you tolerate a maximum of 90 dB peaks, that calculates to 102 dB at a distance of 1 meter from the speaker (halving the distance is a 6dB gain). If the speaker has a sensitivity of 90 dB at one meter, then the power required to raise it 12 dB at one meter is 16 watts (90 dB/1w, 93 dB/2w, 96 dB/4w... ).

However, that would be in a theoretical room. Real rooms have things in it with absorbing and reflecting surfaces that throws a wrench in those calculations. Best go by the speaker mfr’s power rating limits.

(http://www.sengpielaudio.com/calculator-levelchange.htm) (https://thehub.musiciansfriend.com/tech-tips/tech-tip-wattage-speaker-efficiency-amplifier-loudness)  (https://geoffthegreygeek.com/amplifier-power/)

Most stereo systems are played too loud in an attempt to approximate live levels.This only leads to listening fatigue. I like average levels around 70 - 80 db, with peak levels about 90 + Beyond that my ears start to shut down!

A peak level 10 db above average requires 10 x the power in watts, approximately.