It's very rare for regimes to crumble in the aftermath of a significant military victory, so a Bolshevik takeover is off the table. The Bolsheviks' appeal lay largely in their opposition to the war (and Lenin's 'revolution' was more a glorified coup in practice, anyway). For a good while, political dissent of any shade is likely to be marginalized, the Tsar's popular cult effectively able to ride him out of the revolutionary crosshair. The impetus for something politically drastic and immediate just isn't there. Nevertheless, the contradictions inherent to imperial Russia are simply too urgent to sustain Nicholas' rule in the long run, and the masses now have a precedent in the toppling of Kaiser Wilhelm, an autocrat with a style of governance lifted directly from the Romanovs.
I don't foresee any major Marxist involvement in the inevitable upheaval, and I don't even think it would resemble the February Revolution. ATL's revolution is going to be a 'second 1905', with a combination of disorganized urban and rural unrest boiling over into civil society petitioning for economic and political reform along German lines. The biggest issue the Tsar faces is one of his own making - that he's already proven himself a totally unreliable partner, ready to bask in his own absolutist legend at the expense of any semblance of constitutionalism. This isn't going to be like the first revolution, in that his unwillingness to cooperate with reformers is no longer a matter of speculation (see his undermining of the Duma between 1906-1914). Either the whole monarchy is going to be done away (in favour of a government likely dominated by SRs), or a more pliable figurehead (ideally the Tsarevich, in his late-teens around this point) placed on the throne. The only means via which Nicholas can survive this trap would be to demonstrate a lot more political rationale in his dealings with the post-war civilian authorities, and his track record up to this point is not good at all - modesty and pragmatism divorced of naked ideology came about as naturally to him as it did to Adolf Hitler.
From here, we enter uncharted territory. The revolutionary government is likely to face some serious struggles with the military, particularly emboldened in the light of its triumph on the Eastern Front, and other reactionary columns, although the lack of a strong radical leftist bloc, and thereby its perceived influence over the leadership, means tensions won't necessarily reach the excesses they did IOTL, being a mostly behind-the-scenes deal. The nature of the relationship will, at any rate, undermine the Republic's democratic credentials. There will be no instantaneous transfer of clear, hegemonic authority to elected forces, and the quest for a stable Russian constitutionalism of the British or French variety is set to be perilous and by no means successful.