best way to solve volume control in Logic X
What you are talking about is usually called "Gain Staging" and an important technique/workflow from recording to the final mix. Regardless if you record with audio hardware or work on a mixing console or in a DAW or combined. Gain staging is not only a matter of levels. You have to hear the loudness and at the same time watch your meters.
A DAW like Logic has an almost infinitive dynamic range (except for the input and output channels). This is different from analog hardware. However, best practice is still, not to overdrive plugins, channel inputs, sends and sidechains unless you have a good reason for that.
Target is to get maximal output with minimal voltage. This means, EQing and limiting often belongs to gain staging because there are always frequencies and peaks which just push the meters without any musical contribution. A wise EQ and/or a careful limiter as very first plugins can spare you a lot of work. Otherwise you may turn the volume down to acommodate elements you don't want anyway. If you remove them later, the balance gets lost and it is annoying to hop back across several channels to find the source of the problem.
the best way to begin, at least, is to center all the sliders on 0.0
No. There is no rule and no best thing. If you start with a quiet romantic piece at zero, the mix may still be too quiet. If you do this with a metal guitar-drums-orgy, you will find yourself lowering all volume sliders over time until they are halfway down. If you work on a classical or experimental piece with a huge dynamic range, you may not find any system but rather set the volume individually per instrument.
BTW, this zero-fader suggestion comes from analog consoles. There you have a lot of headroom before distortion and you want the level high enough above noise level. Not so in digital systems, where you have no headroom at all and DAWs do not make noise on their own. With the fader at zero in Logic you can only go 6 dB up. Not much room. You can increase your "artificial headroom" by lowering the fader and increase the output to compensate. A monitor controller helps a lot during the mix, finally you set the correct output before bouncing.
Just a recommendation: Keep your output channelstrip plugin-free and at zero. Your channelstrips should send everything to a bus/aux which you call master bus or so. From there to the output. Same for subgroups.
I know not to touch the main volume slider.
Correct. With a little work you can even hide it to avoid unwanted changes. The master volume slider allows to control all outputs at once, but if you have only a stereo output it makes no sense. Better use a decent monitor controller (hardware), leave the DAW output at zero and never touch the master.
change the volume level by going into the Piano Roll and changing the volume there.
Wait a moment – on notes in the piano roll or score you adjust velocity which is different from volume. Velocity tells how hard a string, key or drum is played. For blown instruments it tells the initial force of blowing. Velocity can change the sound dramatically.
If an instrument is too quiet, think about the method to make it louder. Hit it harder (velocity on notes) or turn the volume up or both. Compression can help. Or is it not the right instrument? Natural instruments have their loudness ranges. In reality you can't play an upright bass as loud as a trombone. Increasing the velocity may emotionally not fit while higher volume may sound awkward. Rethink the arrangement.
also have tried the automation volume change
If you mean the track automation then yes, this is an option. Originally it is thought for changing values but you can of course use it to adjust steady levels. I would just not do it for the volume faders, better spare those for final automation.
my rhythm sounds ( piano and guitar ) are ALWAYS much softer than my other sounds
This is correct, ok? A nylon guitar is quieter than a steel guitar which is quieter than a piano which is quieter than a drumset which may be quieter than a strong synthesizer which is quieter than an airplane. Everything you change in this balance will sound unnatural and yes, you want it unnatural. Just think about the method that fits to your musical imagination. Hit the instrument harder or turn the volume up or use compression or limiting (= squeeze it and make it louder afterwards).
Another approach: Several instances of the same or similar instruments playing unisono with slight variations (timing, micropitch, panning) give you a fuller sound.
any help would be appreciated
Many mixing engineers on consoles make a rough mix at first. They set all faders to zero, let the music play and use the trim knobs to get an almost well balanced sound. They keep the faders for mix automation.
Logic does not have trim knobs (unfortunately) but you can do a similar thing: Set your faders to -10 db or so to make your artificial headroom. Insert the Logic Gain plugin as the first or the last plugin in each channelstrip. Got the idea? Fader at -10 gives possible +16 for final automation, the gain plugin in the channel can do +/- 24 dB. Plenty of room. Now trim the volume of your channels with the gain plugins.
Another method is to insert one gain plugin for trimming and a second one for automation of different parts depending on the song structure. The volume faders are still available for final automation.
Hope this helps or at least helps for thinking about gain staging.
