I have owned the ARC SP3A-1 and the SP8 MKII preamps and I can tell you they are totally different. I have not owned the VTL 2.5, but I have owned the VTL Deluxe and the VTL Ultimate preamp. None of these preamps have anything in common with each other. Of these three preamps the VTL Ultimate preamp was my favorite providing you have the right tubes in it. I used military grade Mullard tubes in the line stage and VTL labeled 12AX7s in the phono stage to achieve the best sound from the Ultimate preamp.
With the SP3A-1 I used an ARC D-75 amplifier with Living Voice Auditorium speakers.
With the SP8 MKII I used the matching ARC D-115 MKII power amplifier with Dunlavy SC IV speakers.
With the VTL Ultimate preamp I used VTL Compact 100 mono amplifiers with Martin Logan CLS speakers and Muse Model 18 subwoofer.
The VTL Deluxe preamp was not in the same league and was not taken seriously.
The ARC SP3A-1 is the oldest preamp with a much warmer sound. This preamp does not have a modern sound, but it sounds very good.
The ARC SP8 MKII preamp is by far the most neutral sounding preamp of the group with a highly detailed yet smooth sound. This preamp also has an amazing phono stage. One of the best I have heard.
The VTL Ultimate preamp is my all time favorite preamp. This preamp is a true dual mono design with 2 separate power supplies, 2 power cords, 2 power swithes dual selector switches and the line stage has dual left-right circuits. The phone stage board and tubes are suspended on a trampoline so they are totally isolated. This preamp does it all. It gives a wonderful full bodied sound with detail, dynamics and the biggest soundstage.
The VTL Ultimate sold new for $4000. I recently saw one advertised on Audiogon for $900. It sold instantly.
I have no experience with the VTL 2.5.
I hope this is helpful.
With the SP3A-1 I used an ARC D-75 amplifier with Living Voice Auditorium speakers.
With the SP8 MKII I used the matching ARC D-115 MKII power amplifier with Dunlavy SC IV speakers.
With the VTL Ultimate preamp I used VTL Compact 100 mono amplifiers with Martin Logan CLS speakers and Muse Model 18 subwoofer.
The VTL Deluxe preamp was not in the same league and was not taken seriously.
The ARC SP3A-1 is the oldest preamp with a much warmer sound. This preamp does not have a modern sound, but it sounds very good.
The ARC SP8 MKII preamp is by far the most neutral sounding preamp of the group with a highly detailed yet smooth sound. This preamp also has an amazing phono stage. One of the best I have heard.
The VTL Ultimate preamp is my all time favorite preamp. This preamp is a true dual mono design with 2 separate power supplies, 2 power cords, 2 power swithes dual selector switches and the line stage has dual left-right circuits. The phone stage board and tubes are suspended on a trampoline so they are totally isolated. This preamp does it all. It gives a wonderful full bodied sound with detail, dynamics and the biggest soundstage.
The VTL Ultimate sold new for $4000. I recently saw one advertised on Audiogon for $900. It sold instantly.
I have no experience with the VTL 2.5.
I hope this is helpful.