Bose Headphones

...should have.

Settings/Equalizer (via the "gearwheel", though it may not be in use by default)

There's no Equalizer under the gearwheel.  There is "Music Quality" and I have set it to Very High from Normal under "Streaming".  It has made...absolutely no difference.

It is 'phone dependent, and if it is available changing it may/will change it for other apps.

In the app Home/Settings/Playback/Equalizer

...from the horse's mouth:

support.spotify.com/us/using_spotify/features/high-quality-streaming/

Not sure how you're connecting your 'phone, Al, but given the vintage of the amp, I'd be surprised if it's via bluetooth.

My guess would be out via the headphone socket on the 'phone and in via aux connections on the amp. If so, they well be mismatched. If you can't find an equalizer, then turn the volume on the 'phone to the minimum level that makes the amp volume control still viable for normal listening levels - the circuitry in the amplifier will be much better than the 'phone, and the better balance of amplification may improve things.

If you were planning on streaming Spotify (or anything else via the 'phone) this way for a bit, you might consider a bluetooth receiver for the amp. Just attach to the aux sockets and keep its battery charged, then use Bluetooth to stream. I bought a mid-price Tx or Rx unit from eBay to stream from the main HiFi to my wireless headphones, but they are available below £10 (but check the reviews). The results might be better (and possibly more convenient).

www.ebay.co.uk/itm/2in1-Wireless-Bluetooth5-0-Audio-Transmitter-Receiver-HIFI-Music-Adapter-AUX-RCA/402273930107

...is just an example, I can't vouch for it.

I have a Yamaha WXAD10 unit on the second HiFi which will stream just about anything at decent quality, via bluetooth, WiFi or ethernet, (it also drives the wireless headphones) but I think that's probably pushing the boat out too far for you.

Just to be difficult, I might suggest the opposite, as the phone may control volume through the headphone socket digitally - effectively by throwing away digital information before converting it to analogue. Yes, the Denon amp will have a better analogue stage than the phone, but it may still sound better if you keep the phone output to maximum. This is what I do with Bluetooth speakers - admittedly from an iPhone that no longer has an analogue output stage to worry about.

That should have read "If so, they may well be mismatched"

Amplifier inputs are effectively electronically tuned (impedence, sensitivity, roll-off, etc.) to the type and characteristics of the input source expected to be connected*.

They can generally cope with quite a wide variance in input parameters, but close matching gives the absolute sweet-spot on the sound quality.

A headphone-out to an aux-in isn't really a conventional match (though, depending on the characteristics of both devices, might be alright).

* most noticeably, the "phono" input for a record deck is very different to the others, and contains significant additional "equalisation" circuitry, but in the past, a number of CD players had a higher output voltage than was ideal for the general inputs, and the CD input of an amp might have been rated at a lower sensitivity to match things better.

The specs for your amp indicate it has quite a high sensitivity, (certainly compared with any of mine) which could make it a poorer match for a headphone output from a 'phone.