I'm currently bulk aging 6-gallons of merlot wine which I made from juice purchased from a local winery. It is definitely drinkable but its taste and mouth-feel are boring and flat. I think its missing acid and tannins. Is there anything I can do now or is it too late? Can I add powdered tannin? what about adding citric acid?

Have you measured the acid? The acid should be about .55%. It may be lower than that percentage. Use an acid titration kit to check the acid. Under the chemical section there is some more info about measuring acid.

You can experiment with tannin. It doesn't take much. You might take a glass of the merlot and add a few grains of tannin to the glass to see if it perks up the flavor.

If you decide it needs tannin you have to make a slurry with the tannin first otherwise the tannin will turn into a mudball when adding it to the wine.

I have added both acid and tannin pretty close to bottling time and have good luck with it.

Thanks so much for the reply wyo wino. I was hoping you would; you've got some great insight and info. I don't have an acid kit <yet>, but I love your recommendation of adding tiny amounts of tannin to a glass of wine. I'm going to give that a try. Thanks again.

If you make a few changes it may be a good idea to wait a few weeks before you bottle.

Welcome to the forum.