I have a few additional suggestions (I played in various orchestras for years, taught privately and have been a musical instrument repair tech for almost 17 years. My sister is also a public school orff, choir and band teacher).
It sounds like what you really need to do is memorize the feel of the dynamics (pianissimo, to piano, to mezzo-piano, forte, etc etc) One of the best ways for you to do this might be to get a Db meter and use it to gauge how hard you are hitting. Another way is to ask a hearing friend to listen and tell you when you are first "audible" (meaning pp/ppp) and then monitor you as you increase in volume.
For you it's going to be a FEEL thing ... feel the tension in your arms and hands, feel the vibration, the impact, and look at how the bars rebound.
Also if there are more than one of you playing the same instrument - if they're beside you ask if you can work together privately to not only match volumes, but set up private cues which you can see that indicate you need to play softer, or louder (it could be something as simple as them tilting their head up (for more volume) and down (for less volume).
Also - since you have a set up at home, consider setting it up so you are facing a corner (so your face it pointed at the < of the walls and within 3 or 4 feet) ... this really helps bounce the sound bask towards you so you can better hear volume.
Hope some of this helps 
If you want to bounce ideas of someone, feel free to PM me (I don't check the music section much)
BTW, for anyone interested in common dynamic ranges & their abbreviations here's the list from loudest to softest:"mezzo" is Italian for "medium" or "moderate"
fff ---- louder than ff
ff ----- fortissimo ------ louder than f
f ------ forte -------------- loudly
mf --- mezzo-forte ------ moderately loud
mp -- mezzo-piano ----- moderately soft
p ----- piano -------------- softly
pp --- pianissimo ------ softer than p
ppp - softer than pp
