It shoudln't be hard to acidify your soil a little. For example, amend it with a peat product. If you used Pro-Mix HP, I'd say mix 30% of it with 70% of whatever you're using now. Pro-Mix HP for me is about 5.8-6.0 out of the bag. (I mix it 50-60% to 20-25% soil which is 6.8-7.0 out of the bag and 20-25% perlite. I get about 6.2 as a result. Dolomite raises it to 6.4.).
If you don't want to play with the texture of your soil that way, some people use aluminum sulfate. A tiny amount goes a long way. Try 3g per gallon of medium. That should get you close to 6.5. (However, some feel aluminium should be avoided because it's unhealthy to humans. It's commonly used for blueberries. Peat and organics have metals too. Nothing's pure, except perhaps hydro. So, you'd have to weigh how you feel about this.).