I have 3a sigs, also 2 harbeth models, c7 and shl5 (i have heard the mon 30 plenty but not in my own system, i never liked it enough to own it relative to other harbeth models)
vandy 3a sigs have fuller bass, larger presentation slightly recessed midrange, smooth sophisticated highs (with good solid state amplfiication, which is what is required) - overall a very balanced, imminently listenable full range speaker with no weaknesses but also no towering strengths
mon 30’s have sweet highs and a more prominent luscious full bodied midrange, decent mid bass, but weak true bass... mon 30’s in a full range set up definitely need support from subs imo... their lossy cabinet design and driver quality give a more ’hear through’ tactile midrange than the vandy’s (which have a nice midrange but somewhat less articulate), that is their main strength
both speakers have a sweet slightly rounded off quality to how they play music... not very ’alive’ nor highly impactful 'fast' speakers but that is also what makes them so easy to live with and be happy with, they subtly smooth out bad edgy recordings, they impart no listening fatigue, yet they are transparent and tonally true enough to let well recorded music come through in all its glory
