There are suprisingly not many speaker switches on the market, and only really one that has a good reputation for being transparent (not messing up the sound) which is the Beresford one here:
Beresford TC-7220 MKIII Amplifier/Speaker Selector | eBay
Not that inexpensive at £109.99, but I have used it myself in the past, and it works fine, and I could hear no change in the sound when using it.
I have also used a graphic equaliser before, and found it useful at the time. Adding another device to the audio chain could degrade the sound, but if you can't hear any degradation then don't worry about it, and if you find you can use it to improve the sound, or make it more to your liking, then why not!
There are probably other ways of equalising the sound though, especially if using an AVR. The modern Denon and Marantz ones have an onboard graphic equaliser, as well as Audyssey room correction. But, to use those, you can't have the amp in direct mode. So it is swings and roundabouts as to whether messing with the sound at all is going to make it worse, but then if you can change features of the sound to make it more pleasing then that is the pay off.
Dirac room correction is better than Audyssey, and many people like to use that for music. You can get that via a number of methods, probably the most cost effective solution is the Nad T758v3 AVR here:
NAD T 758 V3 AV Receiver
Ideally, you would want to fix any issues by changing the speakers, placing them differently or treating the room. Not always possible though.
I found the sound quality for music to be very disappointing when I was using a Denon 6300, and a Denon 4400 AVR. It was better with a Marantz SR7011, and much better still with my current Arcam AVR550. Other users are reporting that the Nad that I linked to sounds the same as the Arcam.