A lot of water softeners add fairly high amounts of sodium that can be harmful to the plant, and many of them also remove calcium and magnesium from the water... both of which are essential nutrients. I'd do a light soil flush with pH'ed (preferably regular tap water), or distilled/RO water with CalMag added. Afterwards, give them a light feeding with a complete (and water soluble) fertilizer. Get yourself a quality fertilizer that's higher in nitrogen than in phosphorus and potassium to use if the yellowing worsens. Start with very diluted feedings (only 1/4-1/2 of the recommended strength) and work your way up from there until you start seeing a difference. It would also be a good idea to check and make sure the most badly affected plants haven't become rootbound in their pots. If they have, repot them into larger containers with fresh soil and preferably some extra perlite. Scott's soil is OK for growing, but it's not ideal with the time-released fertilizers and moisture retaining agents. Cutting with extra perlite will greatly enhance the drainage of the soil.

Your Epsoma fertilizer is a good soil amendment, but you'll periodically need to supplement with soluble fertilizers that have either been depleted by the roots, or haven't yet had a chance to break down. I've never used this particular fertilizer, but I have one of their similar products that contained mycorrhizae/beneficial bacteria, and had great results with it. It also had a similar NPK to your Plant Tone, and the plants really seemed to love it.

Also, warm air rises... so it might be a better idea to pull your cooler intake air in from the bottom of your box, and exhaust out the top. If you're able to do so, that is. Try to keep your 'daytime' temps below 80F for best results.