This paper proposes a sufficient condition to assess linearized Bounded-Input-Bounded-Output (BIBO) stability of Multi-Vendor-Multi-Terminal (MVMT) HVDC grid over broad frequencies. Without madatory utilization of Nyquist Criteria or passivity of impedance, this paper provides a guidance on how the stability of an HVDC grid can be predicted based on measurements during the tests on disconnected components without access to internal design of HVDC converters. Assisted by realizing positive realness of an auxiliary transfer matrix, defined as current gain matrix, in frequency domain, a set of sufficient criteria are formulated. They include one global criterion, i.e. positive realness of every current (voltage) gain and two decentralized criteria, i.e. BIBO stability of every nodal impedance (and admittance), and BIBO stability of every transfer admittance (and impedance). The sufficiency of the stability criteria is derived via nodal analysis in s-domain. The paper also presents the schemes of testing stability of nodal admittance/impedance in time-domain to cover converters controlled as constant power loads. The overall methodologies are further generalized to bipolar HVDC grids including unbalanced conditions and transfer network with low-impedance branches. The achievability of positive real voltage/current gains is compared with passive nodal admittance in case studies assisted by time-domain simulations using Matlab/simulink.