Reddit mentions of Rockford RFHLC High Level Speaker Signal to Low Level RCA Adapter

The quick easy way would be to use a converter like this https://www.amazon.com/Rockford-RFHLC-Speaker-Signal-Adapter/dp/B009VTDVMU

The more difficult way would be to open it up and trace the audio path to where it enters the power amplifier section.

Yes, it's quite possible. You need one of these.

OH another option might be is if your amp has "B" speaker outputs. Then you could use something like this: Rockford RFHLC High Level Speaker Signal to Low Level RCA Adapter https://www.amazon.com/dp/B009VTDVMU/

They're made to connect amplifiers to car stereos but it should work just fine for your VU meter. You could connect it in parallel with your existing front speakers as well, but then you'd want to boost the gain of those channels to compensate for the added load.

If the Technics Amp does not have PRE-AMP output, then this gets difficult. It can probably be done, but it is unclear how well it will work.

https://www.hifiengine.com/manual_library/technics/sa-103.shtml

There are Speaker Level to Line Level (RCA) converters that are meant of car stereos. But they might work for you.

https://www.amazon.com/Rockford-RFHLC-Speaker-Signal-Adapter/dp/B009VTDVMU/

https://www.amazon.com/Kicker-KISLOC-2-Channel-Speaker-Converter/dp/B00I4EF1BC/

Though search AMAZON for "speaker level to line level converter" though be aware that it doesn't always return the correct products. But you will see some similar to those above.

If you are willing to seek alternatives, then give us a working budget for those alternatives.

Also important, what is the source of your sound? TV? Turntable? Computer? CD Player? Network Streaming? Other? All of the above? None of the above?

I'm guessing absolute minimum for an alternative solutions is going to be in the $50 to $100 range. But to make those suggestions we need to know what is going into the system in terms of sources of sound. And we need a clear declaration of your working budget.

Yes. It's called a line level converter, $15+. Any reason you want to run it from there?

If the headphone jack isn't working there might be a tape copy output on the back of the receiver that might work as an RCA out for music.

It's a hack, but there are plenty of voltage-divider style adapters that will get you from speaker out to line out. The sound will suffer from the connect:amp's amp, though. Example.

If you want to use your speaker outputs, you'll need something like this: https://www.amazon.com/Rockford-RFHLC-Speaker-Signal-Adapter/dp/B009VTDVMU

Low level input is needed for your Bose system, or any active speaker.

Though, don't use the adapter if you want the sound "parallel" to your normal speakers. If you do that, you'll create a parallel speaker setup, which (if you have two 6 ohm speakers, for example), halves the impedance (so now you have 3 ohms). That can cause an overload on the amp, in the best case ist switches of, in the worst it goes up in flames.
Also, by coupling passive speakers with active ones , you'll create a delay for the active ones, since the signal has to go through the low level converter first and then again through an amplifier. That will cause a delay and will ultimately sound worse than before.

This one outputs 3v max, and it’s cheap

Rockford RFHLC High Level Speaker Signal to Low Level RCA Adapter https://www.amazon.com/dp/B009VTDVMU/ref=cm_sw_r_cp_api_VbbUBbAS5ZMBW

Rockford RFHLC High Level Speaker Signal to Low Level RCA Adapter