Tankless Water heater

I was planning on installing an electric tankless water heater in my home which requires two 240V 60A lines. The closet that containes my water heaters has two independent 240V breakers being fed via 10-3 wire to two 30A breakers in my main panel.

I had planned to just upgrade the two breakers in the main panel to two 60A breakers per the specifications for the unit. Upon further review I see that the required wiring is 6 gauge wiring. I assume the 10-3 wiring I have isn't sufficient.

Now comes the part where I'm trying to figure out my options:

1. Run two brand new 6 gauge cables from the main panel (two 60A) to the spot. Problem here is that it would be a HUGE pain the neck to run that cable.

2. Run the 6 gauge wire with two 60A breakers from a nearby sub panel (wreck room or audio panel). This would be crazy convenient, especially if I can put a third 60A breaker/cable so I can get the better unit. Problem here is I have no idea if the sub panels are sufficient to provide the load and don't know how to calculate that.

3. Install a dedicated sub panel for the tankless heater which would reduce the amount of cables to run and could open up options to expansion and such. Same problem with knowing which panel to run from and how to calculate the proper gauge and/or panel.

I'm just trying to get an idea of what's needed so I can purchase materials and maybe even run the cables and mount the boxes. I'd save the inspection and actual hookup for the electrician but I like to do as much as I can (safely) beforehand.

Panels should have clickable links to pictures. Any suggestions would be greatly appreciated.

Water Heater Closet
Main Panel
Main Panel Meter
Wreck Room Panel
Audio Room Panel
Tankless WH install info

do you have gas to the house you i think that may be your best opp im no sparky but it looks like like you close to full load for that service but i may be wrong.
i have been doing some research on tank less and have found that gas tank less water heaters are better when it comes to heating water than electric but if that all you get to work with thats all you can do i hope someone has the answer you are looking for

Is your main panel large enough to handle this large additional load?
The two inside sub panels do not look even close to large enough for that additional load. The feed is going to need to come from one of the two outside panels IF they are large enough. I can't tell from the pictures.

Not trying to be disrespectful here because I'm all for DIY but, I have a concern with someone doing electrical work who seems to think that all you have to do to get 60 amps on a circuit is to simply change the breaker. I'll acknowledge that you figured out you need to change the wire also but there is an underlying lack of basic electrical know how.
I would get a quote from a couple of electricians that can look at your existing service to see if it has the capacity to even add what you want, and to give you some proper ideas.

I was going to install a tankless electric w.h., but after a lot of research decided not to because
1-hard water soon makes the unit less efficient, so it must be periodicly serviced
2- the pay back time for the unit is about as long as it will last before it has to be replaced, so you never really get the savings you are looking for
3- the initial expense is quite high, even if you do all the work yourself
4-you have to gauge ACCURATELY how much hot water is needed per hour at the time you would use it the most, also, how high of a temperature difference you need from the ambient water temp.To that figure , you need to add 10%.
If you don't gauge it properly, it will be useless
I think that if you are absolutely set on getting one, then gas is the better choice over electric, it's more efficient

oh, yeah, I forgot, go to the forum on "Water Softener and Heater" down near the bottom of the subjects, and click on tankless water heaters, read the reviews and decide if you want one