We're picking at nits here people.

The question: Is it better to mix all instruments to the same level?

What exactly does that mean to the OP? Is the OP planning to mix the drums and the acoustic guitar and the lead electric guitar and the piano to the same level? I would hope not. Or does the OP mean the acoustic guitar on song 1 is recorded at the same level as the acoustic guitar on song 2 and song 5 and song 21 of the set list? Well, even to that question I would have to say, probably not. No 2 songs are the same. The EQ or reverb I use on one song is rarely ever exactly the same as the one I use in the next song I work on. No cookie cutter solutions seem to work for me. So I'm left with this simple fact: Every song is totally different in it's settings. However, that does not relieve me of the task to get my songs all to that same level of quality where I should be able to take any of them, place them into a CD format, pop it in to a player of one kind or the other and play that CD without having to tweek the tone or grab the volume. They should all sound like they were recorded in the same studio, at about the same time, and should sound like a comprehensive project, and not a mashed up bunch of songs with different temperaments that just happened to end up on the same CD. The word is "consistency".

As a (previously) working musician myself, having made my living with music for a very long time, and playing in bands and also gigging solo for a time using my own backing tracks printed to CD and played through a PA system at the gigs and the events, over the course of many years, I have learned a thing or two about how things tend to work in the real world. Not exclusively theory.... I have plenty of real world experience.

I can say, that it is totally possible to use a studio mix that has EQ and overall volume levels of the songs on the backing CD set to the same consistent standards and levels and NOT have to adjust the volume or the EQ settings as one progresses through the sets and from song to song. Every PA I have ever owned, or played through, had the ability to make every single CD (commercial or homegrown) sound good.

The key is to simply have consistency in the recording process and in the mixing stages. In the same way that you would and could take any given commercial CD release from a major artist and play that CD in the same PA system. Once you set the PA channel volume and adjust the EQ as needed for personal taste and to the room characteristics..... nothing should need to be changed from that point on.

My mixes were sufficiently consistent, that once set, the rest of the night was good. It's called a sound check and every musician & band needs to do one. The goal, whether live or backing CD, is to have the mix on the CD to the point that the tone controls should be fairly close to center 0. If the EQ...hi, mid, or low needs to be increased or decreased from that center 0db flat point..... the mix from the studio isn't right to start with. But again.... if all the songs are consistently in the same EQ ballpark.... hey, it's consistent, and if the tone controls can make the adjustment and the result sounds good....it's all good and the musician should NOT have to tweek knobs during the show.

This is really not a discussion about mixing in the studio vs mixing through a PA....(we have had that discussion just a few weeks back)...it's about being able to achieve consistency in the work you are outputting from your studio. That, for many people, is very hard, if not impossible to achieve, especially when starting out. It's for that reason, not being consistent in the mixing, that this problem arises, not from where the mix is done. (studio vs PA) If the recording was not done (tracking) with consistency, you will still have problems.

Studio monitors should give the superior or more properly said.....accurate mix. Studio monitors are designed to give a flatter response, and thereby a more accurate rendering of what is really happening in the mix across the spectrum. In addition, the room, in many studios has some level of acoustic treatment.

PA speakers are not designed in many cases, to studio standards. The speakers in many PA systems have a "brand name" sound to them or a bias of some sort. So, the sound you hear is not accurate and mixing based on that will give you a skewed result.

The solution to the problem is simple: Consistency at all stages of the process from one song to the next. There is a learning curve which may be years long before true consistency is achieved. I'm still not where I want to be in this respect.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.