With plug-ins you can get loud sound, but it will be ugly, squashed, sharp and distorted as hell. With analog masteringsound can be really loud, but pleasant to ears!
This happens because home computer mastering works only with digital numbers and gain reduction/compression is done only with virtual digits. This cause digital unpleasant distortion if high volume is used. During real mastering we are working with true analog signal and various methods of natural sounding gain reduction as: transparent analog compression, vacuum tube saturation, tape saturation of trebles and bass, analog limiting, clipping of hi-end AD converters (much better sounding than digital limiters, technique used on all today's commercial albums), precise transparent digital limiting with 16x oversampling. By combination of various analog and digital gain reduction you can get loud/modern sound but without distortion, squashed sound, ultra sharp and harsh middles, harsh trebles and transients but with pleasant and dynamic fat sound.
Digital mastering produce sharp distorted transients if competitive (high) volume is used, but analog is much more natural and smooth.
Home computer mastering will be probably cold and boring, with analog simulation plug-ins used on master it will be distorted and maybe terribly harsh if high RMS volume level is used. Analog vs. Digital in mastering is like driving car in computer game vs. driving real sport car.
Digital mastered bass is usually squared not round and fat like analog. It is almost impossible to make same warm and fat bass in digital domain like during real analog mastering.
Small distortion, unpleasant frequencies and harshness in sound is multiplied many times when music is reproduced at big PA system in club, festival or hi-end audiophile hifi ! So if computer mastering doesn't sound good on small speakers/monitors on big PA it will be disaster.
Simulation of vacuum tubes and other analog devices in computer usually creates distortion and aliasing instead of warm sound and real color.
This happens because of digital distortion and fact that analog devices are creating higher harmonics, that means that fundamental frequency is multiplied. In digital domain (simulation of vacuum tube anlog gear) if 16 kHz frequency has higher harmonics, they are = 32kHz (2nd harmonics), 48khz (3rd harmonics) etc. but CD audio is limited up to 44,1 kHz sampling rate, so what happens if these higher harmonics are generated in digital world? Signal above 44,1 is usually returned back as ugly crisp digital artefacts / aliasing! This do not happen in analog world.
Your small studio monitors/speakers are good for mixing, checking frequencies and frequency conflicts, but has flat cold sound not suitable for mastering. Also you cannot hear deepest sub bass frequencies with standard monitors. This is fact. All mastering studios are using detailed, transparent hi-end audiophile speakers that can reproduce colour and real character of sound. Next fact is: you probably don't have 4 pairs of speakers and headphones in studio. If music sounds good with monitors it may not with various hifi speakers. Main goal of mastering is to make music sounding good everywhere. We have various hi-end to low-end speakers for checking.
If you don't have mathematically calculated and measured acoustics of your studio you can expect more than +/- 6db frequency distortion / unreal hearing of sound. If you bought some diffusers and foam it can help but not for mastering because foam can absorb many frequencies more than desired.
PLEASE NOTE: There is big difference between use of plug-ins on individual channels/mixing and using plug-ins on stereo master bus (mastering) which works with complex signal (many channels mixed into 2 stereo tracks left and right) so while VST compressor/equalizer may sound fine on individual channel like guitar, synthesizer etc. it can sounds really harsh and unpleasant on stereo master bus where plug-in must works with very detailed complex information (often more than 20 channels mixed together). For this reasons all professional producers who are mixing ITB are using external analog mastering studios.